ADAPTIVE IMMUNE RESPONSE When the body is exposed to an antigen Innate Immunity Adaptive Immunity (Acquired)

Active immunity Passive Immunity •B cells encounter antigens and produce antibodies • Active immunity can be naturally or artificially acquired Passive Immunity • Antibodies are obtained from someone else • Conferred naturally from a mother to her fetus • Conferred artificially from immune serum or gamma globulin • Immunological memory does not occur • Protection provided by “borrowed antibodies”

Types of Adaptive Immune Response 1 Types of Adaptive Immune Response 1. Cell-mediated immune response (CMIR) – T cells 2. Humoral immune response – B cells

Humoral Immune Response B lymphocytes with specific receptors bind to a specific antigen  The binding event activates the lymphocyte to undergo clonal selection  A large number of clones are produced (primary humoral response)  Most B cells become plasma cells Produce antibodies to destroy antigens Activity lasts for four or five days  Some B cells become long-lived memory cells (secondary humoral response)

Activation of B cells Recognition of antigen by B cells B cells recognize their cognate antigen in its native form. They recognize free antigens in the blood or lymph using their B-cell receptor (BCR) or membrane bound-immunoglobulin After antigen recognition, the B cell ingests the whole protein antigen and processes it into peptides for presentation to activated T cells.

Antibodies Proteins that recognize and bind to a particular antigen with very high specificity. Made in response to exposure to the antigen. One virus or microbe may have several antigenic determinant sites, to which different antibodies may bind. Each antibody has at least two identical sites that bind antigen: Antigen binding sites. Valence of an antibody: Number of antigen binding sites. Most are bivalent. Belong to a group of serum proteins called immunoglobulins (Igs).

held together by disulphide bridges Antibody Structure Monomer: A flexible Y-shaped molecule with four polypeptide chains: 2 identical light chains 2 identical heavy chains Variable Regions: Two sections at the end of Y’s arms. Contain the antigen binding sites (Fab). Identical on the same antibody, but vary from one antibody to another. Constant Regions:Stem of monomer and lower parts of Y arms. Fc region: Important because they can bind to complement or cells. held together by disulphide bridges

Antibody Structure

Hinge Region The region at which the arms of the antibody molecule forms a Y is called the hinge region because there is some flexibility in the molecule at this point.

An antibody digested by papain yields three fragments: two Fab fragments and one Fc fragment

General Functions of Immunoglobulins Antigen binding (primary function) Immunoglobulins bind specifically to one or more antigenic determinant and can result in protection of the host.  Valency: Is the number of antigenic determinants that an individual antibody molecule can bind. The valency of all antibodies is at least two and in some instances more.

Effector functions Fixation of complement lysis of cells, release of biologically active molecules Binding to various cells: phagocytic cells, lymphocytes, mast cells, and basophils have receptors that bind immunoglobulins and thus get activated. Some immunoglobulins also bind to receptors on placental trophoblasts, leading to transfer of the Igs across the placenta (provide immunity to the fetus)

Immunoglobulin Classes IgG - Gamma (γ) heavy chains IgM - Mu (µ) heavy chains IgA - Alpha (α) heavy chains IgD - Delta (δ) heavy chains IgE - Epsilon (ε) heavy chains Immunoglobulin Subclasses IgG Subclasses IgG1 - Gamma 1 (γ1) heavy chains IgG2 - Gamma 2 (γ2) heavy chains IgG3 - Gamma 3 (γ3) heavy chains IgG4 - Gamma 4 (γ4) heavy chains IgA subclasses IgA1 - Alpha 1 (α1) heavy chains IgA2 - Alpha 2 (α2) heavy chains Immunoglobulin Light Chain Types: Kappa (κ) Lambda (λ)

Different Immunoglobulins

Number of Ag binding sites Type Number of Ag binding sites Site of action Functions IgG 2 Blood Tissue fluid Can cross placenta Increase macrophage activity Antitoxins Agglutination IgM 10 IgA 2 or 4 Secretions (saliva, tears, small intestine, vaginal, prostate, nasal, breast milk) Stop bacteria adhering to host cells Prevents bacteria forming colonies on mucous membranes IgE Tissues Activate mast cells  HISTAMINE Worm response IgD B-cell surfaces where it functions as a receptor for Ag. B cell activation Cε4