1

AN ASSIGNMENT

ON

IMMUNE EFFECTOR MECHANISM

SUBMITTED TO

Dr. Monira Noor

Assistant Professor

Department of Pathology

Faculty of Veterinary and Animal Science

Sylhet Agricultural University, Sylhet-3100

SUBMITTED BY

DR. Muhammed Hossain

M.S Student

ID. No. 1401011201

Registration No. 0445

Session: January-June’2014

Department of Parasitology

Faculty of Veterinary and Animal Science

Sylhet Agricultural University, Sylhet-3100

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INDEX

SERIAL NO. CONTENTS PAGE NO.

01 Introduction 01-02

02 General Discussion 03-30

03 Conclusion 31

04 References 32-35

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Introduction

Immune system may be defined as the system which deals with the cells and

molecules that function is to protect against any infection. Historically, the immune

system was separated into two branches: Humoral immunity for which the protective

function of immunization could be found in the humor (cell-free bodily fluid or

serum) and cellular immunity, for which the protective function of immunization was

associated with cells. CD4 cells or helper T cells provide protection against different

pathogens. Cytotoxic T cells cause death by apoptosis without using cytokines;

therefore in cell mediated immunity cytokines are not always present.Humoral

immunity also called the antibody-mediated immune system is the aspect of immunity

that is mediated by macromolecules, as opposed to cell-mediated immunity, found in

extracellular fluids such as secreted antibodies, complement proteins and certain

antimicrobial peptides. Humoral immunity is so named because it involves substances

found in the humors or body fluids.The study of the molecular and cellular

components that comprise the immune system including their function and inter-

action is the central science of immunology. The immune system is divided into a

more primitive innate immune system and acquired or adaptive immune system of

vertebrates, each of which contains humoral and cellular components. It also refers to

the effector functions of antibodies, which include pathogen and toxin neutralization,

classical complement activation and opsonin promotion of phagocytosis and pathogen

elimination.

The immune system distinguishes two groups of foreign substances. One group

consists of antigens that are freely circulating in the body. These include molecules,

viruses, and foreign cells. A second group consists of self cells that display aberrant

MHC proteins. Aberrant MHC proteins can originate from antigens that have been

engulfed and broken down (exogenous antigens) or from virus‐infected and tumor

cells that are actively synthesizing foreign proteins (endogenous antigens).Cell-

mediated immunity is an immune response that does not involve antibodies but rather

involves the activation of phagocytes, antigen-specific cytotoxicT-lymphocytes and

the release of various cytokines in response to an antigen. Antibody-mediated

neutralization of microbes and toxins requires only the antigen-binding regions of the

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antibodies. Therefore, such neutralization may be mediated by antibodies of any

isotype in the circulation and in mucosal secretions and can experimentally also be

mediated by Fab fragments of specific antibodies, which lack the Fc regions of the

heavy chains. Most neutralizing antibodies in the blood are of the IgGisotype; in

mucosal organs, they are largely of the IgA isotype. The most effective neutral izing

antibodies are those with high affinities for their antigens. High-affinity antibodies are

produced by the process of affinity maturation. Many microbes enter host cells by the

binding of particular microbial surface molecules to membrane proteins or lipids on

the surface of host cells. For example, influenza viruses use their envelope

hemagglutinin to infect respiratory epithelial cells, and gram-negative bacteria use pili

to attach to and infect a variety of host cells. Antibodies that bind to these microbial

structures interfere with the ability of the microbes to interact with cellular receptors

by means of steric hindrance and may thus prevent infection. Many microbial toxins

mediate their pathologic effects also by binding to specific cellular receptors. For

instance, tetanus toxin binds to receptors in the motor end plate of neuromuscular

junctions and inhibits neuromuscular transmission, which leads to paralysis, and

diphtheria toxin binds to cellular receptors and enters various cells, where it inhibits

protein synthesis. Antibodies against such toxins sterically hinder the interactions of

toxins with host cells and thus prevent the toxins from causing tissue injury and

disease.

Objectives:

1. To know details about immune systems.

2. To know how the immune system acts against microbes.

3. To know the detail procedure of microbes neutralization.

4. To know the mechanism of antigen neutralizations by the process of humoral

and cell mediated immunity.

5. To understand how CD4+ and CD8+ T cells function to eliminate intracellular

microbes in different cellular compartments in the effector phase of immune

responses.

6. To understand the properties and functions of the Th1, T22 and Th17 subsets of

CD4+ effector T cells.

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General Discussion

The study of the molecular and cellular components that comprise the immune

system, including their function and interaction is the central science of immunology.

For the action immune system it is important to development of immunity in the body.

Development of the Immune Mechanism

Fig: Development of the immune mechanism

Humoral Immunity:

Humoral immunity also called the antibody-mediated immune system, is the aspect of

immunity that is mediated by macromolecules as opposed to cell-mediated immunity,

found in extracellular fluids such as secreted antibodies, complement proteins and

certain antimicrobial peptides. Humoral immunity is so named because it involves

substances found in the humours or body fluids.

Humoral immunity refers to antibody production and the accessory processes that

accompany it, including: Th2 activation and cytokine production, germinal center

formation and isotype switching, affinity maturation and memory cell generation. It

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also refers to the effector functions of antibodies, which include pathogen and toxin

neutralization, classical complement activation and opsonin promotion of

Phagocytosis and pathogen elimination.

Another name of adaptive immunity is acquired immunity. It may be two types which

is given below:

Fig:Humoral and cell-mediated immunity

The humoral response (or antibody‐mediated response) involves B cells that recognize

antigens or pathogens that are circulating in the lymph or blood. The response follows

this chain of events:

1. Antigens bind to B cells.

2. Interleukins or helper T cells co-stimulate B cells. In most cases, both an

antigen and a co-stimulator are required to activate a B cell and initiate B cell

proliferation.

3. B cells proliferate and produce plasma cells. The plasma cells bear antibodies

with the identical antigen specificity as the antigen receptors of the activated B

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cells. The antibodies are released and circulate through the body, binding to

antigens.

4. B cells produce memory cells. Memory cells provide future immunity.

Major discoveries in the study of humoral immunity

Substance Activity Discovery

Alexin(s)

Complement

Soluble components in the serum

that are capable of killing

microorganisms

Buchner (1890),

Ehrlich (1892)

Antitoxins

Substances in the serum that can

neutralize

the activity of toxins, enabling passive

immunization

von Behring and Kitasato

(1890)

Bacteriolysins

Serum substances that work with the

complement proteins to induce bacterial

lysis

Richard Pfeiffer (1895)

Bacterial

agglutinins

and precipitins

Serum substances that agglutinate

bacteria

and precipitate bacterial toxins

von Gruber and Durham

(1896),

Kraus (1897)

Hemolysins

Serum substances that work with

complement

to lyse red blood cells

Belfanti and Carbone

(1898)[8]

Jules Bordet (1899)

Opsonins

Serum substances that coat the outer

membrane

of foreign substances and enhance the

rate of

phagocytosis by macrophages

Wright and Douglas

(1903)

Antibody

Formation (1900), antigen-antibody

binding Founder: P Ehrlich

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hypothesis (1938), produced by B cells

(1948),

structure (1972), immunoglobulin genes

(1976)

Difference between Humoral Immunity and Cell Mediated Immunity:

Humoral Immunity Cell Mediated Immunity

1. Destroy extra cellular microbes.

2. Responding B lymphocytes.

3. Effector mechanism secreted Ab.

4. Transferred by serum.

5. Block infections and eliminate

extra cellular microbes.

1. Phagocytosis microbes in

macrophage or intra cellular.

2. Responding T lymphocytes.

3. Effector mechanism does secreted

Ab.

4. Transferred by lymphocytes.

5. Activate MQs and eliminate

reservoirs of infections.

Antibody

Immunoglobulins are glycoproteins in the immunoglobulin superfamily that function

as antibodies. The terms antibody and immunoglobulin are often used

interchangeably. They are found in the blood and tissue fluids as well as many

secretions. In structure, they are large Y-shaped globular proteins. In mammals there

are five types of antibody: IgA, IgD, IgE, IgG, and IgM. Each immunoglobulin class

differs in its biological properties and has evolved to deal with different antigens.

Antibodies are synthesized and secreted by plasma cells that are derived from the B

cells of the immune system.

Structure of Antibody:

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Fig: Structure of antibody

Antibody Opsonization

Antibody opsonization is the process by which a pathogen is marked for ingestion and

destruction by a phagocyte. Opsonization involves the binding of an opsonin, e.g.,

antibody to an epitope on an antigen. After opsonin binds to the membrane,

phagocytes are attracted to the pathogen. The Fab portion of the antibody binds to the

antigen, whereas the Fc portion of the antibody binds to an Fc receptor on the

phagocyte, facilitating phagocytosis. The receptor-opsonin complex can also create

byproducts like C3b and C4b which are important components of the complement

system. These components are deposited on the cell surface of the pathogen and aid in

its destruction.The cell can also be destroyed by a process called antibody-dependent

cellular cytotoxicity, in which the pathogen does not need to be phagocytosed to be

destroyed. During this process, the pathogen is opsonized and bound with the antibody

IgG via its Fab domain. Then the antibody binds an immune effector cell via its Fc

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domain and this binding triggers a release of lysis products from the bound immune

effector cell (monocytes, neutrophils, eosinophils and natural killer cells). This

process can cause inflammation of surrounding tissues and damage to healthy cells.

Characteristics of Different Antibody Classes

1. IgG is the major antibody produced in response to secondary and higher-order

antigen encounters. Secondary immune responses are faster and longer and

produce more antibodies with higher affinity to the antigen compared with

primary responses. Hence, IgG is the most prevalent antibody in serum and is

responsible for adaptive immunity to bacteria and other microorganisms. IgG

exists in serum as a monomer. There are four IgG subclasses in humans (IgG1,

2, 3, and 4). IgG has a half-life of 23 days, the longest amongst the antibody

classes. When bound to antigen, IgG can activate serum complement and cause

opsonization. It can cross the placenta and is secreted into colostrum,

protecting the fetus as well as the newborn from infection.

2. IgA usually exists as a polymer of the fundamental Y-shaped antibody unit. In

most IgA molecules, a joining chain (J chain) holds together two antibody units

(dimeric form). As the IgA passes through epithelial cells, an additional

antigenic fragment, the secretory piece, is added. In this conformation, IgA is

actively secreted into saliva, tears, colostrum, and mucus and hence is known

as secretory immunoglobulin (sIgA). IgA is also found in serum, mainly as

IgA1 isotype produced by bone marrow B cells, while IgA2 subtype is present

in secretions. IgA has a half-life of about 6 days.

3. IgM consists of five “Y” units, also held together by J chains. The size of IgM

and its many antigen-binding sites provide the molecule with an excellent

capacity for agglutination of bacteria and blood cells. Although it has the

potential to bind 10 antigens, in reality it only binds five. Fixed macrophages of

the RES system efficiently and quickly remove such agglutinated antigens.

IgM is the first antibody secreted after an initial immune challenge. It has a

half-life of about 5 days.

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4. IgE is a monomeric antibody that is slightly larger than IgG but has a relatively

short half-life of about 3 days. IgE avidly binds via its Fc region to cells, such

as mast cells and basophils, which are involved in allergic reactions.

5. IgD is found in plasma and on the surface of some immature B cells. It has a

half-life of 3 days. An exact function is not known, but its ubiquitous presence

in the animal kingdom suggests an important role. It is expressed early during

B-cell differentiation and has been postulated to be involved in the induction of

immune tolerance. IgD was also found to activate basophils and mast cells. IgD

serum concentration does increase during chronic infection but is not

associated with any particular disease.

Antibody action:

Antibodies act against antigens in three ways: they neutralize the antigen, opsonize the

antigen, or stimulate complement fixation to assist phagocytes.

1. Neutralization:Antibodies can bind to antigens, forming easily recognizable

antibody–antigen complexes, which are removed by phagocytosis. Antibodies

can also immobilize and agglutinate infectious agents so that a virus cannot

penetrate the host cell or a microbe cannot colonize mucosal tissue.

2. Opsonization:IgG antibodies bind to bacteria or virus-infected cells at the Fab

region. That way, the pathogen is “tagged” or “opsonized” for destruction by free

radicals and enzymes or phagocytosis. For phagocytosis, the Fc portion of the

antibody binds to Fc receptors on phagocytes. Some complement components

(e.g., C3b and C4b) can also act as opsonins.

3. Complement Fixation: Complement is a group of at least nine distinct proteins

that circulate in plasma, but involves about 25 proteins and protein fragments. A

cascade of events occurs when the first protein, C1, recognizes preformed IgM or

IgG antibody–antigen complexes (classical pathway). A side entry to this

pathway exists, in which complexes between bacterial mannose residues and the

plasma protein mannose-binding lectin start the cascade (lectin pathway). The

cascade leads to the formation of C3 convertase through the action of C4 and C2,

and so the convertase has the form C4b2a.

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Humoral Immune Effector Mechanism:

Neutralization of microbes and microbial toxins

Antibody-Mediated Opsonization and Phagocytosis

Antibodies of the IgGisotype coat (opsonize) microbes and promote their

phagocytosis by binding to Fc receptors on phagocytes. Mononuclear phagocytes and

neutrophils ingest microbes as a prelude to intracellular killing and degradation. These

phagocytes express a variety of surface receptors that directly bind microbes and

ingest them, even without antibodies, providing one mechanism of innate immunity

(see Chapter 4). The efficiency of this process is markedly enhanced if the phagocyte

can bind the particle with high affinity. Mononuclear phagocytes and neutrophils

express receptors for the Fc portions of IgG antibodies that specifically bind antibody-

coated (opsonized) particles. Microbes may also be opsonized by a product of

complement activation called C3b and are phagocytosed by binding to a leukocyte

receptor for C3b (described later in this chapter). The process of coating particles to

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promote phagocytosis is called opsonization, and substances that perform this

function, including antibodies and complement proteins, are called opsonins.

Antibody dependent cellular cytotoxicity:

Natural killer cells and other leukocyte bind to antibody coated cells and destroy these

cells.

Fig: Antibody dependent cellular cytotoxicity.

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Complement mediated cytolysis:

The complement system is a biochemical cascade of the innate immune system that

helps clear pathogens from an organism. It is derived from many small blood

plasmaproteins that work together to disrupt the target cell's plasma membrane leading

to cytolysis of the cell. The complement system consists of more than 35 soluble and

cell-bound proteins, 12 of which are directly involved in the complement pathways.

The complement system is involved in the activities of both innate immunity and

acquired immunity. Activation of this system leads to cytolysis, chemotaxis,

opsonization, immune clearance, and inflammation, as well as the marking of

pathogens for phagocytosis. The proteins account for 5% of the serumglobulin

fraction. Most of these proteins circulate as zymogens, which are inactive until

proteolytic cleavage.

Fig: Complement mediated cytolysis

Three biochemical pathways activate the complement system: the classical

complement pathway, the alternate complement pathway, and the mannose-binding

lectin pathway. The classical complement pathway typically requires antibodies for

activation and is a specific immune response, while the alternate pathway can be

activated without the presence of antibodies and is considered a non-specific immune

response.[1]

Antibodies, in particular the IgG1 class, can also "fix" complement.

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The complement can be also used for purification of a particular cell population. As

you learnt earlier, each cell population expresses a unique set of surface antigens. By

treatment of a cell mixture with complement plus antibodies specific for the surface

antigens of one of the cell populations, this cell population will be specifically

eliminated from the cell mixture. By using different combinations of the antibodies,

all cells in a mixture except the cell type of interest can be removed, leaving a single,

isolated cell type viable for further study. This technique is called antibody-mediated

cytolysis.

Fig: B cell activation is a large part of the humoral immune response.

The principal function of B cells is to make antibodies against soluble antigens. B cell

recognition of antigen is not the only element necessary for B cell activation (a

combination of clonal proliferation and terminal differentiation into plasma cells).

B cell activation depends on one of three mechanisms: Type 1 T cell-independent

(polyclonal) activation, Type 2 T cell-independent activation (in which mature B cells

respond to highly repetitive structures causing cross-linking of the B cell receptors on

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the surface of B cells), and T cell-dependent activation. During T cell-dependent

activation, an antigen presenting cell (APC) presents a processed antigen to a helper T

(Th) cell, priming it. When a B cell processes and presents the same antigen to the

primed Th cell, the T cell releases cytokines that activate the B cell.

Humoral immunity is mediated by secreting antibodies: B cells (with co-

stimulation) transform into plasma cells that secrete antibodies. Binding of antigen to

the BCR activates B lymphocytes, which then start proliferating and maturing in the

presence of T helper cytokines (Fig. 10.4). In the classical, somewhat outdated model,

B-cell–activating cytokines, such as IL-4, are part of the T helper 2 response. Most

new cells become plasma cells, which produce antibodies for 4 to 5 days, resulting in

a high level of antibodies in plasma and other body fluids. These antibodies can bind

specifically to the antigenic determinant that induced their secretion. Other B-cell

clones become long-lived memory B cells. In contrast to the time-delayed response of

cell-mediated immunity, antibodies are known to induce immediate responses to

antigens and, thereby, provoke immediate hypersensitivity reactions.

Fig: Humoral Immunity by B cells

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Principal cell types in immune system:

1). Lymphocytes come from a distinct lineage of stem cells and are responsible for the

adaptive response. Lymphocytes are the only cells in the body that are capable of

recognizing specific antigens through an array of highly specific receptors.

Lymphocytes develop from stem cells in the bone marrow and differentiate into three

populations of cells that are important in immune responses: B cells, T cells and

natural killer (NK) cells. B cells are the only cells capable of producing antibodies and

are the cells that mediate humoral immunity. On the other hand, T cells are involved

in cell-mediated immunity. Helper T cells secrete cytokines that help B cells to mount

potent antibody responses and assist macrophages in killing phagocytosed microbes.

Cytotoxic T cells directly kill infected cells. Like cytotoxic T cells, NK cells also kill

infected or damaged cells but have receptors that are much less specific. NK cells are

considered part of innate immunity while B and T cells participate in adaptive

immunity.

2) Antigen presenting cells are capable of bringing antigens to lymphocytes to initiate

the adaptive response.The main job of antigen presenting cells (APCs) is to capture

and display antigens to lymphocytes. Most APCs are located in the periphery where

they survey tissues for antigens. Unlike lymphocytes, which recognize very specific

antigens, an APC has receptors that recognize broad classes of microbial antigens.

When stimulated, these receptors cause the stimulating antigen to be endocytosed.

This initiates several responses by the APC that occur simultaneously: the APC

displays the antigen on its surface via MHC II molecules; the APC synthesizes

molecules that facilitate its migration into the lymph and to a local lymph node; the

APC synthesizes and displays additional molecules on its surface that will act as co -

activators of lymphocytes in the lymph node. The best understood antigen presenting

cell is the dendritic cell. Other cells in the body that possess the special MHC II

molecules needed to display endocytosed antigens are macrophages and B cells. Thus,

macrophages and B cells can also act as antigen presenting cells.

3) Effector cells actually do the killing of microbes once an adaptive response is

underway. The cells that actually eliminate the microbe are called effector cells. This

group is made up of activated lymphocytes and other leukocytes. Activated helper T

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cells stimulate macrophages to kill endocytosed microbes; activated cytotoxic T cel ls

and NK cells directly kill infected cells; plasma cells (antibody-secreting B cells)

secrete antibodies, which target antigens for destruction. Non-lymphoid leukocytes

such as macrophages and granulocytes participate in the elimination of microbes in

both the innate and adaptive immune responses.

Cell Mediated Immunity

Cell-mediated immunity is an immune response that does not involve antibodies, but

rather involves the activation of phagocytes, antigen-specific cytotoxicT-lymphocytes,

and the release of various cytokines in response to an antigen. Historically, the

immune system was separated into two branches: humoral immunity, for which the

protective function of immunization could be found in the humor (cell-free bodily

fluid or serum) and cellular immunity, for which the protective function of

immunization was associated with cells. CD4 cells or helper T cells provide protection

against different pathogens. Cytotoxic T cells cause death by apoptosis without using

cytokines, therefore in cell mediated immunity cytokines are not always present.The

following chain of events describes this immune response:

1. Self cells or APCs displaying foreign antigens bind to T cells.

2. Interleukins (secreted by APCs or helper T cells) co-stimulate activation of T

cells.

3. If MHC‐ I and endogenous antigens are displayed on the plasma membrane, T

cells proliferate, producing cytotoxic T cells. Cytotoxic T cells destroy cells

displaying the antigens.

4. If MHC‐ II and exogenous antigens are displayed on the plasma membrane, T

cells proliferate, producing helper T cells. Helper T cells release interleukins

(and other cytokines), which stimulate B cells to produce antibodies that bind

to the antigens and stimulate nonspecific agents (NK and macrophages) to

destroy the antigens.

Cellular immunity protects the body by:

1. Activating antigen-specific cytotoxic T-lymphocytes that are able to induce

apoptosis in body cells displaying epitopes of foreign antigen on their surface,

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such as virus-infected cells, cells with intracellular bacteria, and cancer cells

displaying tumor antigens;

2. Activating macrophages and natural killer cells, enabling them to destroy

pathogens.

3. Stimulating cells to secrete a variety of cytokines that influence the function of

other cells involved in adaptive immune responses and innate immune

responses.

Cell-mediated response involves activating T cells and releasing cytokines:

Cell-mediated responses in adaptive immunity do not involve antibodies or

complement but, rather, activation of macrophages, NK cells, T lymphocytes, and the

release of various cytokines in response to an antigen.T cells continuously patrol the

body and check the foreignness of antigen. T cells become activated when antigen

binds to the specific TCR plus a costimulatory element. Antigen can only bind to the

TCR when presented by APCs in combination with MHC proteins. The complex

interaction between the T cell and the APC is called immunologic synapse.

Effector mechanism of cell mediated immunity

CD4+ T cells of the Th1 subset secrete the cytokine IFN-γ:Which activates

macrophages to killphagocytosed (vesicular) microbes. Recall that an importantearly

reaction of host defense (a component ofinnate immunity) is phagocytosis of

microbes. Manypathogenic microbes are able to resist the killing abilityof

macrophages and survive and even replicate inmacrophages. In order to kill these

microbes, thephagocytes have to be activated by T cells (illustratinghow adaptive

immunity provides more powerful defensethan innate immunity). Macrophage killing

mechanisms are largely restricted to vesicles (to protectthe macrophage itself from

destruction). CD4+ effectorT cells of the Th1 subset stimulate the

lysosomalmicrobicidal mechanisms of macrophages and thus promotethe destruction

of phagocytosed microbes. Thesame cellular responses give rise to delayed type

hypersensitivity(DTH), the pathologic consequence of the reactionwhen it is directed

against self antigens oragainst microbes that resist eradication.

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Fig: CD4+ T cells of the Th1 subset secrete the cytokine IFN-γ

CD4+ T cells of the Th2 subset secrete IL-4 and IL- 13, which stimulate the

production of IgE antibody,and IL-5, which activates eosinophils, important for

the elimination of helminths. These cytokines also activatemast cells, and they

promote immunity at mucosalbarriers by blocking microbe entry and promoting

expulsion.

CD4+ T cells of the Th17 subset secrete IL-17,which stimulates other cells to

produce chemoattractantcytokines (chemokines) that recruit leukocytes(mainly

neutrophils and some monocytes) from theblood. This inflammatory reaction is

important for defenseagainst fungal and bacterial infections, and contributesto many

chronic inflammatory diseases.

The role of CD8+ T cells in host defense:CD8+ T cells kill infected cells

containing cytoplasmicmicrobes; CD8+ T cells can also activate phagocytes.CD8+

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CTLs recognize class I MHC-associated peptideantigens on antigen-expressing

(typically, infected ortumor) cell → degranulation of CTLs → release of

perforin,which → promotes entry of granzymes (granule enzymes)into the infected

cell → granzymes cleave andactivate caspases → apoptosis of infec ted cell.CD8+

CTLs are also involved in killing of tumors andin rejection of tissue transplants.

Phases of CMI:

Effector and memory cells are generated by stimulationof naïve T cells by a microbe.

Subsequent exposure tothat microbe (in the same infection or in a repeat

infection)triggers the following sequence of reactions:

1. Migration of effector T cells from site of generation(peripheral lymphoid

organs) to site of infection(any tissue).

2. Retention of antigen-stimulated T cells at site of infection.

3. Activation of effector T cells at the site of infection,resulting in the eradication

of infections

T cell migration and retention:

1. Effector T cells express high levels of integrins andligands for E- and P-

selectins.

2. Cytokines produced at site of infection promote expressionof ligands for

integrins, and E- and P- selectins,on endothelial cells. Therefore,

circulatingeffector T cells bind to endothelium at sites of infection.

3. Chemokines produced by tissue macrophages andother cells (in response to

microbes) attract T cellsto site of infection.

4. T cells that encounter antigens in extravascular tissuesexpress adhesion

molecules that attach to extracellularmatrix proteins, such as high-affinity

integrins(e.g. VLA-4, which binds to fibronectin)and CD44 (binds to

hyaluronate). Therefore,antigen-specific T cells are preferentially retainedat the

site of antigen (i.e. infection), giving themtime to do their job.The pathways

and mechanisms of lymphocyte migrationand homing were discussed in the

lecture on FunctionalAnatomy of the Immune System.

T cell mediated macrophage activation:

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1. Mediated mainly by TH1 cells

2. Upon activation by microbial antigens, CD4+ T cells express CD40 ligand

(CD40L) and secrete cytokines

3. CD40L on the T cells engages CD40 on macrophages Cytokines, notably IFN γ

produced by Th1 cells,bind to IFN-γ receptors on macrophages

4. Signals from CD40 and IFN-γR activate the macrophagesto produces

microbicidal substances (reactiveoxygen species, nitric oxide, lysosomal

enzymes),and other mediators of inflammation(such as cytokines). This type of

macrophage activation,which functions mainly to enhance the

microbicidalactivities of the macrophages, is called“classical” macrophage

activation.

5. CD4+ Th1-mediated macrophage activation caneliminate microbes that are

confined to vesiclesbut is ineffective against microbes that haveevolved to

escape from vesicles into the cytoplasm.In such infections, CD8+ CTLs kill the

infectedcells. Thus, both classes of T cells cooperate to combatintracellular

infections.

Other T Cell Subsets

1. Th17 cells stimulate the production of chemokinesin tissues that recruit more

phagocytes (neutrophilsand monocytes), as part of an inflammatoryreaction.

These recruited cells ingest and destroyextracellular microbes (bacteria and

fungi).

2. Th2 cytokines activate macrophages not to becomebetter killers of

phagocytosed microbes but to terminateinflammation and produce growth

factorsand enzymes that start the process of tissue repair.This type of

macrophage activation is called “alternative”macrophage activation.

CD4+ and CD8+ mediated destruction of microbes:

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Fig: CD4+ and CD8+ mediated destruction of microbes

Effectors’ CD4+ T cell mediated immunity

Fig: Effectors’ CD4+ T cell mediated immunity

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T helper cell

The T helper cells (Th cells) are a type of T cells that play an important role in the

immune system, particularly in the adaptive immune system. They help the activity of

other immune cells by releasing T cell cytokines. They are essential in B cellantibody

class switching, in the activation and growth of cytotoxic T cells, and in maximizing

bactericidal activity of phagocytes such as macrophages.

Mature Th cells express the surface protein CD4 and are referred to as CD4+ T cells.

CD4+ T cells are generally treated as having a pre-defined role as helper T cells within

the immune system. For example, when an antigen presenting cell expresses an

antigen on MHC class II, a CD4+ cell will aid those cells through a combination of

cell to cell interactions (e.g. CD40 and CD40L) and through cytokines. Nevertheless,

there are rare exceptions; for example, sub-groups of regulatory T cells, natural killer

T cells, and cytotoxic T cells express CD4 (although cytotoxic examples have been

observed in extremely low numbers in specific disease states, they are usually

considered non-existent). All of the latter CD4+ T cell groups are not considered T

helper cells.

Activation of naive helper T cells

Following T cell development, matured, naïve T cells leave the thymus and begin to

spread throughout the body, including the lymph nodes. (Naïve T cells are those T

cells that have never been exposed to the antigen that they are programmed to respond

to). Like all T cells, they express the T cell receptor-CD3 complex. The T cell receptor

(TCR) consists of both constant and variable regions. The variable region determines

what antigen the T cell can respond to. CD4+ T cells have TCRs with an affinity for

Class II MHC, and CD4 is involved in determining MHC affinity during maturation in

the thymus. Class II MHC proteins are generally only found on the surface of

specialisedantigen-presenting cells (APCs). Specialised antigen presenting cells are

primarily dendritic cells, macrophages and B cells, although B cells are the only cell

group that expresses MHC Class II constitutively (at all times). Some APCs also bind

native (or unprocessed) antigens to their surface, such as follicular dendritic cells, but

unprocessed antigens do not interact with T cells and are not involved in their

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activation. The antigens that bind to MHC proteins are always short peptides, 8-10

amino acids long for MHC Class I, and up to 25 or so for MHC Class II.

Th1 Cells

Th1 cells are involved in the cellular immune response and host defense against

intracellular pathogens. These molecules are characterized by the production of pro-

inflammatory cytokines like IFN-γ, IL-2, and lymphotoxin-α (LTα). Th1 cells are

centrally involved in cell-mediated immunity. The cytokines produced by Th1 cells

stimulate the phagocytosis and destruction of microbial pathogens by macrophages

and other lymphocytes. Several chronic inflammatory diseases have been described as

Th1-dominant diseases including multiple sclerosis, diabetes, and rheumatoid arthritis.

Th2 Cells

Th2 cells are involved in the humoral immune response and host defense against

extracellular parasites. These cells are characterized by the production of IL-4, IL-5,

IL-6, IL-10, and IL-13. Th2 cells are thought to play a role in allergic responses.

Cytokines like IL-4 generally stimulate the production of antibodies directed toward

large extracellular parasites, while IL-5 stimulates eosinophil response toward large

extracellular parasites. Allergy and atrophy are thought to be Th2-dominant

conditions. Th2 cells have historically been thought to be the source of IL-9.

However, recent publications suggest the existence of a Th2-related cell type that is

characterized by the secretion of IL-9 and IL-10. These so-called Th9 cells can

differentiate from Th2 cells in the presence of TGF-β, or they can differentiate from a

naïve CD4 cell with a combination of IL-4 and TGF-β. These cells may be involved in

asthma and tissue inflammation.

Th17 cells

Th17 cells are involved in inflammation and host defense against extracellular

pathogens. A subset of helper T cells that produce IL-17A, Th17 cells has been shown

to play an important role in the induction of autoimmune tissue injury. They are

distinct from Th1 or Th2 cells since they do not produce classical Th1 or Th2

cytokines such as IFN-γ or IL-4. They play a key role in mouse models of

autoimmunity, and it has been suggested that the differentiation pathway from a naïve

T-helper cell to a Th17 cell involves a combination of TGF-β and IL-6. RORγt is a

26

key transcription factor involved in induction of Th17 cells. Some RORγt expression

is induced in response to IL-6 or TGF-β, but the generation of Th17 cells requires

TGF-β, as well as IL-6.

Furthermore, it is believed that the relative balance of IL-6 and TGF-β in steady state

would tilt the balance in favor of either Th17 or Treg differentiation in diverse tissues.

Induction of the Th17 subset requires TGF-β and IL-6, while amplification of IL-

17A–producing cells is dependent upon TGF-β and IL-21. Maintenance of a Th17

response primarily depends on IL-23 (p19/p40). IL-23 binds to the IL-23 receptor that

triggers downstream activation of STAT3 and subsequent up regulation of ROR-γ and

production of IL-17A.Since IL-17A leads to the induction of many pro-inflammatory

factors such as TNF, IL-6, and IL-1β, it has been suggested that Th17 cells might be

responsible for the development and/or progression of autoimmune diseases.

Th17 Cells Dominate Autoimmune Inflammatory Responses

IL-23 and IL-17 have been clearly associated with a number of human autoimmune

diseases, validating earlier observation from mouse models. In rheumatoid arthritis

(RA), IL-17 directly acts on stromal cells to promote inflammation by inducing IL-6,

IL-8 and TNF production from synovial cells. Expression of IL-17, as well as TNF

and IL-1, was found to predict joint pathogenesis, while IFNg was protective. The

direct destructive effect of Th17 cells in RA revealed that this subset of Th cells also

induces osteoclastogenesis. The pathogenic role of IL-17 in promotion of bone

destruction/resorption is independent of TNF and IL-1. In multiple sclerosis (MS),

both CD4- and CD8- IL-17-producing cells have been identified in active lesions in

the brain of patients, while the receptors for IL-17 are present on inflamed

endothelium in MS lesions, suggesting that IL-17 may potentiate migration of

inflammatory T cells across the blood-brain barrier.

In opticospinal forms of multiple sclerosis, intrathecal activation of IL-17 and IL-8,

which is induced by IL-17 and is a strong chemoattractant for neutrophils, induces

heavy neutrophil infiltration, which contributes to extensive spinal cord lesion

formation. In psoriasis, the characteristic Th17 signals such as RORgt, IL-17, IL-22,

and IL-23 mRNA are all elevated in affected skin. In inflammatory bowel disease,

expression of IL-17A mRNA and intracellular IL-17A protein are both elevated in the

27

intestinal mucosa of patients. Both Th17 cytokine and Th17/Th1 cytokine-producing

cells are found in the gastrointestinal (GI) tract of patients with Crohn's disease, and

both cell types express IL-23R, CCR6, and the transcription factor RORgt.

Intriguingly, Th17 cells generated by the stimulation of TGFb plus IL-6 include a

subset of cells that produce IL-10 along with IL-17, suggesting the existence of a

down-regulating feedback mechanism in the inflammatory Th17 response.

Cytotoxic T cell

Antigen presentation stimulates T cells to become either "cytotoxic" CD8+ cells or

"helper" CD4+ cells.A cytotoxic T cell (also known as TC, cytotoxic T lymphocyte,

CTL, T-killer cell, cytolytic T cell, CD8+ T-cells or killer T cell) is a T lymphocyte (a

type of white blood cell) that kills cancer cells, cells that are infected (particularly

with viruses), or cells that are damaged in other ways.

Most cytotoxic T cells express T-cell receptors (TCRs) that can recognize a specific

antigen. An antigen is a molecule capable of stimulating an immune response, and is

often produced by cancer cells or viruses. Antigens inside a cell are bound to class I

MHC molecules, and brought to the surface of the cell by the class I MHC molecule,

where they can be recognized by the T cell. If the TCR is specific for that antigen, it

binds to the complex of the class I MHC molecule and the antigen and the T cell

destroys the cell.

Killer, helper and memory T cells

Killer T cells, helper T cells, and memory T cells can be distinguished based on their

immunologic function. Killer T cells, or cytotoxic T cells (CD8+), are lymphocytes

that release lymphotoxins and are designed to kill cells that are infected with viruses

or other pathogens.

Helper T cells (CD4+) help determine which types of immune response the body will

take to attack a particular pathogen. These cells have no cytotoxic activity and do not

directly kill infected cells or clear pathogens. Instead, helper T cells control the

immune response by directing other cells to perform these tasks. They function to

regulate both the innate and adaptive immune responses. Helper T cells direct the

immune response by secreting lymphokines. They stimulate proliferation of B cells

and cytotoxic T cells, attract neutrophils, and activate macrophages. The

28

differentiation of CD4+ T-helper cells into best-known subtypes T helper 1 and helper

2 cells occurs after activation in the peripheral lymphoid system. Each subtype

produces a distinct set of effector molecules.For instance, T helper 1 cells release the

macrophage-activating effector molecules interferon-γ and tumor necrosis factor-α

(TNF-α). The resultant actions, called T helper 1 response, or type 1 response, support

activities of macrophages and cytotoxic T cells of the cellular immune system.On the

other hand, T helper 2 cells produce effector molecules, such as interleukin 4, 5, 13,

25, 31, and 33, among numerous other cytokines. This T helper 2 response, or type 2

response, promotes the actions of B cells and hence the humoral immune system.

Responses of T cells in infectious diseases:

Fig: Response of T cells in infectious diseases

During an immune response, professional antigen-presenting cells (APCs) endocytose

foreign material (typically bacteria or viruses), which undergoes processing, then

29

travel from the infection site to the lymph nodes. Once at the lymph nodes, the APC

begin to present antigen peptides that are bound to Class II MHC, allowing CD4+ T

cells that express the specific TCRs against the peptide/MHC complex to activate.

When a Th cell encounters and recognizes the antigen on an APC, the TCR-CD3

complex binds strongly to the peptide-MHC complex present on the surface of

professional APCs. CD4, a co-receptor of the TCR complex, also binds to a different

section of the MHC molecule. These interactions bring these proteins closer together,

allowing the intracellularkinases present on the TCR, CD3 and CD4 proteins to

activate each other via phosphorylation. With the assistance of a phosphatase present

on the intracellular section of CD45 (common leukocyte antigen), these molecules

activate major Th cell intracellular pathways. These active pathways are known as

Signal 1 of T cell activation, as it is the first and primary pro-activation signal in a Th

cell. Upon subsequent encounters with a given antigen, memory T cells are re-

activated using the same TCR pathways.The binding of the antigen-MHC to the TCR

complex and CD4 may also help the APC and the Th cell adhere during Th cell

activation, but the integrin protein LFA-1 on the T cell and ICAM on the APC are the

primary molecules of adhesion in this cell interaction.

Differentiation of Th1 and Th2 cells

Type 1/ Th1 Type 2/ Th2

Main

partner cell

type

Macrophage B-cell

Cytokines

produced

interferon-γ and TNF-β.

(Interleukin-2 was classically

associated with Th1 cells, but this

association may be misleading; IL-2

is produced by all helper T cells

early in their activation.)

interleukin-10 production has been

shown to be induced in activated

interleukin-4, interleukin-5,

interleukin-6, interleukin-10,

interleukin-13

30

Th1 cell.

Immune

stimulation

promoted

Cellular immune system.

Maximizes the killing efficacy of

the macrophages and the

proliferation of cytotoxic CD8+ T

cells. Also promotes the production

of opsonizing antibodies

Humoral immune system. Stimulates

B-cells into proliferation, to induce B-

cell antibody class switching, and to

increase neutralizing antibody

production.

Other

functions

The Type 1 cytokine IFN-γ

increases the production of

interleukin-12 by dendritic cells and

macrophages, and via positive

feedback, IL-12 stimulates the

production of IFN-γ in helper T

cells, thereby promoting the Th1

profile. IFN-gamma also inhibits the

production of cytokines such as

interleukin-4, an important cytokine

associated with the Type 2 response,

and thus it also acts to preserve its

own response.

The Type 2 response promotes its

own profile using two different

cytokines. Interleukin-4 acts on helper

T cells to promote the production of

Th2 cytokines (including itself; it is

auto-regulatory), while interleukin-10

(IL-10) inhibits a variety of cytokines

including interleukin-2 and IFN-γ in

helper T cells and IL-12 in dendritic

cells and macrophages. The combined

action of these two cytokines suggests

that once the T cell has decided to

produce these cytokines, that decision

is preserved (and also encourages

other T cells to do the same).

While we know about the types of cytokine patterns helper T cells tend to produce, we

understand less about how the patterns themselves are decided. Various evidence

suggests that the type of APC presenting the antigen to the T cell has a major

influence on its profile. Other evidence suggests that the concentration of antigen

presented to the T cell during primary activation influences its choice. The presence of

some cytokines (such as the ones mentioned above) will also influence the response

that will eventually be generated, but our understanding is nowhere near complete.

31

Th17 helper cells

Fig: Immune response by Th17 cells

Th17 helper cells mediate host immunity against extracellular bacteria and fungi. This

effector cell subtype is triggered by IL-6 and TGF beta. Its main effector cytokines are

IL-17a, IL-21, and IL-22. The main Th17 effector cells are neutrophils as well as

IgM/IgA B cells, and IL-17 CD4 T cells. The key Th17 transcription factors are

STAT3 and RORg. TNF alpha can activate neutrophils to kill extracellular bacteria

and fungi. Besides, IL-6 canupregulate the complement system to directly kill

extracellular bacteria and fungi. Th17 overactivation against autoantigen will cause

type 3 immune complex and complement-mediated hypersensitivity. Rheumatoid

arthritis or Arthus reaction belong to this category.[4]

Role of helper T cells in disease

Considering the diverse and important role helper T cells play in the immune system,

it is not surprising that these cells often influence the immune response against

disease. They also appear to make occasional mistakes, or generate responses that

would be politely considered non-beneficial. In the worst case scenario, the helper T

32

cell response could lead to a disaster and the fatality of the host. Fortunately this is a

very rare occurrence.

Helper T cells and hypersensitivity

The immune system must achieve a balance of sensitivity in order to respond to

foreign antigens without responding to the antigens of the host itself. When the

immune system responds to very low levels of antigen that it usually shouldn't

respond to, a hypersensitivity response occurs. Hypersensitivity is believed to be the

cause of allergy and some auto-immune disease.

Hypersensitivity reactions can be divided into four types:

1. Type 1 hypersensitivity includes common immune disorders such as asthma,

allergic rhinitis (hay fever), eczema, urticaria (hives) and anaphylaxis. These

reactions all involve IgEantibodies, which require a Th2 response during helper

T cell development. Preventive treatments, such as corticosteroids and

montelukast, focus on suppressing mast cells or other allergic cells; T cells do

not play a primary role during the actual inflammatory response. Type 2 and

Type 3 hypersensitivity both involve complications from auto-immune or low

affinity antibodies. In both of these reactions, T cells may play an accomplice

role in generating these auto-specific antibodies, although some of these

reactions under Type 2 hypersensitivity would be considered normal in a

healthy immune system (for example, Rhesus factor reactions during child-

birth is a normal immune response against child antigens). The understanding

of the role of helper T cells in these responses is limited but it is generally

thought that Th2 cytokines would promote such disorders. For example, studies

have suggested that lupus (SLE) and other auto-immune diseases of similar

nature can be linked to the production of Th2 cytokines.

2. Type 4 hypersensitivity, also known as delayed type hypersensitivity, are

caused via the over-stimulation of immune cells, commonly lymphocytes and

macrophages, resulting in chronic inflammation and cytokine release. T cells

play an important role in this hypersensitivity, as they activate against the

stimulus itself and promote the activation of other cells; particularly

macrophages via Th1 cytokines.

33

Conclusion

Humoral immunity is mediated by antibodies produced by long-lasting plasma cells

after a first exposure or by reactivation of memory B cells, and is responsible for

defense against extracellular microbes and toxins. The effector functions of antibodies

include neutralization of antigens though Fab region and Fc receptor-dependent

phagocytosis of opsonized particles, and activation of the complement system.

Attachment of antigen-complexedIg to phagocyte Fc receptors delivers signals that

stimulate the microbicidal activities of phagocytes. The biological functions of the

complement system include opsonization of organisms and immune complexes by

proteolytic fragments of C3, followed by binding to phagocyte receptors for

complement fragments and phagocytic clearance, activation of inflammatory cells by

proteolytic fragments of complement proteins called anaphylatoxins, cytolysis

mediated by MAC formation on cell surfaces, solubilizationadn clearance of immune

complexes, and enhancement of humoral immune response. Protective immunity in

neonates is a form of passive immunity provided by maternal antibodies transported

across the placenta by a specialized neonatal Fc receptor T cell-mediated immunity

includes priming of naïve T cells, effector functions of activated T helper cells and

CTL, and long-term persistence of memory T cells. Development of an effective

immune response requires proper activation of T lymphocytes by APC in secondary

lymphoid organs and migration of the responding T cells to the sites of Ag presence in

the body. The efficiency of T cell activation in lymphoid organs depends on the

concentration of an antigenic peptide and affinity of TCR toward the Ag/MHC

complex, and is facilitated by inflammatory stimuli, co-stimulatory signals, and

cytokines. Only a small population of Ag-specific memory cells remains in lymphoid

organs and throughout the tissues for a prolonged period after the immune response is

over. When exposed to the Ag a second time, memory cells rapidly acquire and

mediate effector functions, thereby preventing spread of pathogenic infection.

34

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