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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