ADAPTIVEIMMUNE RESPONSE

When the body isexposed to an antigen

Innate Immunity

Adaptive Immunity(Acquired)

Active 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 Response1 .Cell-mediated immune

response (CMIR) – T cells2. Humoral immune response – B cells

Humoral Immune ResponseB lymphocytes with specific receptors bind to aspecific antigen The binding event activates the lymphocyte toundergo clonal selection A large number of clones are produced (primaryhumoral 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 cellsRecognition 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).

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 ImmunoglobulinsA. 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.

B. 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 chainsImmunoglobulin 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

ImmunoglobulinLight Chain Types:Kappa )κ( Lambda )λ(

Different Immunoglobulins

TypeNumber of Ag binding sites

Site of actionFunctions

IgG2•Blood

•Tissue fluid

•Can cross placenta

•Increase macrophage activity

•Antitoxins

•Agglutination

IgM10•Blood

•Tissue fluid

Agglutination

IgA2 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

IgE2Tissues•Activate mast cells

HISTAMINE

•Worm response

IgD2B-cell surfaces where it functions as a receptor for Ag.

•B cell activation

Cε4