What is Immunity?

Immunity and Disease

■ Immunity is the ability of organisms to defend themselves against pathogens and toxins.

■ Infection is the harmful colonisation of an organism by another species.

Pathogens

■ A pathogen is an organisms that causes disease.

■ Many pathogens are microbes

■ Pathogens infect and cause harm

(Natural)

(Natural)

General Defence System

Non-specific Defence: measures to prevent entry of all pathogens

Skin

■ Physical barrier of dead cells (epidermis)

■ Antimicrobial substances in sebum■ Acidic nature of sebum and sweat■ Blood clotting■ Sebum moisturises which prevents

cracking of skin (would allow entry).

Digestive system■Antimicrobial in saliva■HCl in stomach kills bacteria■Symbiotic bacteria in colon

protect against pathogenic bacteria

Breathing System

■Hairs and mucus in nasal passage

■Mucus in bronchi contains antimicrobial

■Cilia move mucus containing trapped foreign bodies up and out of respiratory tract.

Phagocytic White Blood Cells

■ Phagocytes are special defence cells that feed like amoeba

■ They engulf and digest material (e.g. a virus or bacterium) – phagocytosis

■ Macrophages are large aggressive phagocytes.

■ There are many phagocytes present in the lymph nodes.

Fever■ Raised temperature following

infection■ Increases ability to defeat

pathogensInterferon■ Produced by virus-infected cells■ Increases resistance of

neighbouring non-infected cells

Specific Defence System

How our body can tell the difference between self and non-self

Antibodies and Antigens

■ Antigens are non-self chemicals that stimulate the production of antibodies

■ Antibodies are specific proteins formed in response to the presence of an antigen – antibody binds specifically to the

antigen

How an antibody operates/works?

Deactivation of a bacterium by an antibody.

Monocytes

■ Make up 75% of the white blood cells

■ Some leave the blood and enter tissue becoming macrophages – a type of phagocyte

■ Once they have engulfed antigen, they “present” the antigen on their surface

■ Lymphocytes monitor monocytes for any signs of foreign antigens.

■ Monocyte action is accelerated by Helper T-cells

Phagocytosis

Engulfing germ

Germ

MonocyteGerm

Lymphocytes

■ Specialised white blood cells■ Able to distinguish between self and

non self■ Produced in the bone marrow■ Huge numbers in lymphoid tissue

– spleen, tonsils, adenoids, thymus, intestinal wall

LYMPHATIC SYSTEM

Lymphatic Vessels

One-Way Valves

Tonsils

Thymus

Spleen

Immunity

Active Induced Immunity■ Lymphocytes are activated by antigens

on the surface of pathogens■ Antibodies are produced by the

lymphocytes

Natural active immunity - acquired due to infectionArtificial active immunity – vaccination

■ Takes time for enough B and T cells to be produced to mount an effective response.

Passive Induced Immunity

■ B and T cells are not activated and plasma cells have not produced antibodies.

■ The antigen doesn’t have to be encountered for the body to make the antibodies.

■ Antibodies appear immediately in blood but protection is only temporary.

Artifical Passive ImmunityArtificial passive immunity■ Used when a very rapid immune

response is needed e.g. after infection with tetanus.

■ Human antibodies are injected. In the case of tetanus these are antitoxin antibodies.

■ Antibodies come from blood donors who have recently had the tetanus vaccination.

■ Only provides short term protection as abs destroyed by phagocytes in spleen and liver.

Natural Passive Immunity

Natural Passive immunity■ A mother’s antibodies pass across

the placenta to the foetus and remain for several months.

■ Colostrum (the first breast milk) contains lots of IgA which remain on surface of the baby’s gut wall and pass into blood

Vaccination■ A vaccine is a preparation which gives artificial

active immunity to a pathogen■ May contain live or dead inactivated pathogen or

part of the pathogen.■ This provides a harmless first encounter with the

antigens of the potential pathogen

Benefits of Vaccination■ Provides immunity without suffering

the symptoms of the disease■ Gives long-term protection■ No long-term drug-taking required.

Examples of vaccines:MMR (Measles, mumps and rubella)3 in 1 (whooping cough, diptheria and tetanusBCG (tuberculosis)

HIGHER LEVEL

B-lymophocytes■ Produced and mature in the red

bone-marrow of the foetus■ Migrate to the lymphoid tissue■ If stimulated by antigen, a B-

lymphocyte cell multiplies and differentiates into huge numbers of:

Plasma cells – producing antibodiesMemory B cells – remain after infection

➢ B lymphocytes attack antigens in the blood or body fluids, by producing antibodies that surround the target.

➢ B lymphocytes move to the lymph nodes, where they acquire their receptor molecules.

B LYMPHOCYTES:

➢ Each B cell produces just one type of antibody.

➢ The plasma cells produce antibody molecules, that bind to pathogens and toxins that are circulating in tissues or body fluids, and mark them for destruction by monocytes.

➢ When a B cell encounters matching antigen, they divide into plasma and memory cells.

Memory B-cells■Give immediate protection

against future infections by the same pathogen or antigen

■ If they later detect a previous invader they rapidly reproduce

■This produces a large population of antibody-secreting plasma cells

➢ Memory B and T cells that form during the first response to an antigen, do not engage in that first battle.

➢ They circulate in the body for years, and can intercept antigens far quicker.

➢ This is how we achieve immunity against many diseases – either by getting the disease once or by preventing them using vaccines.

Long-term immunity

T-lymphocytes

■ Also known as T-cells■ Also produced in the foetal bone

marrow, but mature in the thymus■ Migrate to the lymphoid tissue■ Ignore free antigen - act against

virus-infected cells and cancerous cells

■ Multiply and differentiate rapidly when antigen binds to them

Helper T-cells■ Secrete chemicals which turn

the specific defence system “on” and “off”

■ The Helper T-cells– stimulate the production of B-cells– stimulate the formation of Killer T-

cells– accelerate the action of

phagocytes

Killer T-cells

■ Destroy – virus-infected cells– tumour cells– organ-transplant cells (“non-self”)

■ Secrete perforin (punches holes)■ Can stimulate target cell into

apoptosis (programmed cell death or “cell suicide”)

Destruction of Cancer Cell by a Killer T-Cell

Suppressor T-cells■ Maintain level of immune

response■ Inhibit

– B cells– T cells– Monocytes

■ Suppressor T’s stop the immune response when the infection has been defeated

Memory T-Cells• Like memory B-cells they survive a

long time and can respond to a specific invader in the futureMemory T-cells stimulate memory B-cells to start producing antibodies and they stimulate killer T-cells

Summary of T-cellsT-cells do not produce antibodies instead they act in one of four processes:

1. Helper T cells which recognise antigens, enlarge, andsecrete chemicals, such as interferon, which stimulate theproduction of B cells.

2. Killer T cells which attack cells containing a foreignantigen, secrete a chemical called perforin that perforatesthe membranes of cells

3. Suppressor T cells which stop immune responses

4. Memory T cells which can memorise the immunity, even

for life.

Summary of Lymphocytes

Lymphocyte interactions

■ B cell surface antibodies bind antigen

■ Helper T cells also bind, releasing interleukins

■ These chemicals stimulate B cells to multiply and differentiate

■ T cells encountering monocytes that are presenting antigen are stimulated to multiply and differentiate

Human Immunodeficiency Virus

Human Immunodeficiency Virus