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Title
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chapter 1:Introduction to
the immunesystem
Dr. Hafez Sumairi
Learning outcomes
1.What types of immune responses protectindividuals from infections?
2.What are the important characteristics ofimmunity, and what mechanisms are responsiblefor these characteristics?
3.How are the cells and tissues of the immune systemorganized to find and respond to microbes in waysthat lead to their elimination?
The nomenclature, general properties, andcomponents of the immune system• Immunity is defined as resistance to disease, specifically infectious
disease.• Immune system is called the collection of cells, tissues, and
molecules that mediate resistance to infections• Immune response is the coordinated reaction of these cells and
molecules to infectious microbes• Immunology is the study of the immune system and its responses to
invading pathogens.• The physiologic function of the immune system is to prevent
infections and to eradicate established infections
Innate and Adaptive Immunity
• Host defense mechanisms consist of• Innate immunity (also called natural or native immunity) refers to the fact
that this type of host defense is always present in healthy individuals, preparedto block the entry of microbes and to rapidly eliminate microbes that dosucceed in entering host tissues.
• Adaptive immunity (also called specific or acquired immunity) is the type ofhost defense that is stimulated by microbes that invade tissues, that is, it adaptsto the presence of microbial invaders.
Types of adaptiveimmunity1. Humoral immunity
• B lymphocytes• Antibodies
• Circulation andmucosal fluids
• Neutralize andeliminate microbesand microbial toxins
2. Cell-mediated immunity• T lymphocytes
• T helper cell• Cytotoxic cell
Types of Adaptive Immunity
• Active immunity• Immunity may be induced in an individual by infection or vaccination
• Passive immunity• Conferred on an individual by transfer of antibodies or lymphocytes from an
actively immunized individual• Rapid but short term conferring immunity• Newborns
• A naive• Individual, not previously exposed to that microbe’s antigens.
Properties of adaptiveimmune responses
1. Specificity
2. Diversity
3. Memory
4. Clonal expansion
5. Specialization
6. Contraction andhomeostasis
7. Non reactive to self
Properties of adaptiveimmune responsesClonal selection• Mature lymphocytes with receptors for
many antigens develop beforeencounter with these antigens.
• A clone refers to a population oflymphocytes with identical
• Antigen receptors• Specificities• Derived from one precursor cell
Clonal expansion• When lymphocytes are activated by
antigens, they undergo proliferation,generating many thousands of clonalprogeny cells, all with the same antigenspecificity
Properties of adaptiveimmune responses
• Primary immune response, ismediated by lymphocytes,called naive “immunologicallyinexperienced,” lymphocytes,that are seeing antigen for thefirst time
• Secondary immuneresponses,• More rapid, larger, and better
able to eliminate the antigen• Memory lymphocytes,
which are long-lived cells
Cells of the immunesystem
Lymphocytes• The only cells that
produce specificreceptors for antigens
• Distinguishable bysurface proteins thatmay be identified usingpanels of monoclonalantibodies
• CD (cluster ofdifferentiation)
• Numerical designation
Tissues of the immune system
Lymphocytes
Lymphocytes
Antigen-presenting cells (APCs)
• The common portals of entry for microbes the skin, GIT, and RTspecialized APCs located in the epithelium that capture antigens,transport them to peripheral lymphoid tissues, and display them tolymphocytes.
Peripheral lymphoidorgans• Lymph nodes are
nodular aggregatesof lymphoid tissueslocated alonglymphatic channelsthroughout the body
• Spleen• Mucosal and
cutaneous immunesystems
Mucosal immune system
Segregation of T and Blymphocytesin different regions ofperipheral lymphoidorgans.
Lymphocyterecirculation andmigration into tissues• Naive lymphocytes
constantly recirculatebetween the blood andperipheral lymphoidorgans, where they maybe activated by antigensto become effector cells,
• Effector lymphocytesmigrate to sites ofinfection, wheremicrobes are eliminated
Overview of immune responses to microbes
The capture and display of microbialantigens1. Peptides antigen are displayed on the surface of the APCs bound to
MHC molecules
2. Naive T cells recognize these peptide-MHC complexes
3. B lymphocytes recognize protein, Polysaccharides and other non-protein antigens that captured in the lymphoid organs
4. Polysaccharide antigens also results in the activation of complement
5. Naïve T cells are activated to express molecules called co-stimulatorsand to secrete cytokines
6. Transcription of various genes, which encode cytokines, cytokinereceptors, effector molecules, and proteins that control cell cycling
Cell-mediated immunity: activation of Tlymphocytes and elimination of cell-associatedmicrobes• Naïve CD4+ T cells become helper T cells (Th cells)
• Stimulate B cells and macrophages function• Recruit and activate neutrophils
• Naïve CD8+ T cells become Cytotoxic T lymphocytes (CTLs)• CTLs directly kill cells harboring microbes in the cytoplasm
Humoral immunity: activation of Blymphocytes and elimination of extracellularmicrobes• B lymphocytes proliferate and then differentiate into plasma cells that secrete
different classes of antibodies with distinct functions.• Many polysaccharide and lipid antigens acts directly• Protein antigens requires help from CD4+ T cells• B cells ingest protein antigens, degrade them, and display peptides bound to
MHC molecules for recognition by helper T cells.• Polysaccharides and lipids stimulate secretion mainly of a class of antibody
called immunoglobulin M (IgM)• Protein antigens stimulate helper T cells, which induce the production of
antibodies of different classes (IgG, IgA, and IgE) all with the same specificity, iscalled heavy chain class (isotype) switching
• Affinity maturation improves the quality of the humoral immune response.• Neutralizing, opsonize and activate a system of serum complement
Decline of immune responses andimmunological memory• Homeostasis is returning of returning the immune system to its basal
resting state by effector lymphocytes apoptosis after the microbe iseliminated
• Memory cells are an expanded pool of antigen-specific lymphocytesand respond faster and more effectively
• This is why the generation of memory cells is an important goal ofvaccination
Thank you