Formation and function of antibodies

Antibodies are necessary in providing immunity against pathogens and assisting other parts of the immune system

Antibodies

 * Antibodies are gamma globulins (20% of total plasma proteins)
 * Each B-cell clone makes antibody molecules with a specific antigen-binding site
 * The antigen-binding site binds to the membrane and serve as receptors for antigens specific to its site
 * When an antigen binds to the receptors of the antibody on the B-cell, the B-cell activates to multiply and mature into memory cells or into an antibody-secreting cells, which produce antibodies with the same antigen-binding site
 * Antibodies are made of light and heavy polypeptide chains - each heavy chain is paralleled by a light chain at one of its ends
 * The light chain and part of the heavy chains form the variable region; the remainder of the heavy chains form the constant region
 * The variable region is different in each specific antibody and attaches specifically to an antigen
 * The constant region determines the other properties of the antibody e.g: diffusivity in tissues, adherence, attachment to the complement complex, etc.
 * Each antibody is specific to a particular antigen due to complementary amino acid sequences
 * Different parts of the variable regions are produced due to separate gene segments (VDJ sequences), which are recombinated during B-cell differentiation

IgG

 * They are bivalent antibodies and comprise 75% of all antibodies
 * They are produced during the secondary response, identifies micro-organisms for phagocytosis, activates complement system and binds to macrophage receptors
 * IgG-coated foreign cells are attacked by killer cells
 * IgG can pass from mother to foetus via the placenta

IgA

 * They are present in secretions (milk, saliva, tears, respiratory and intestinal secretions)
 * They are dimeric
 * They agglutinate infectious agents in secretions as well as having anti-viral action

IgM

 * They have 10 binding sites
 * Activates complement
 * All B-cells initially make IgM antibodies
 * Stimulates phagocytosis and complement reaction

IgE

 * They have 10 binding sites and bind to receptors on the surface of mast cells and basophils
 * They trigger the secretion of serotonin and histamine by these cells, which increase the permeability of vessels for leukocytes, antibodies and complement components during inflammation

IgD

 * They are rarely secreted by B-cells

Functions of Antibodies
Due to the multivalent nature of the antibodies and the multiple antigen sites on most pathogens, antibodies can inactivate antigens in many ways:-
 * Agglutination - bacteria are bonded through their antigens into a clump
 * Precipitation - soluble antigens and the binded antibodies become insoluble and precipitate
 * Neutralisation - antibodies cover the toxic site of the antigen
 * Lysis - few antibodies are potent enough to directly attack the pathogen membrane and rupture the cell

Effects of the Complement System

 * When an antibody binds with an antigen, a specific site on the constant region of the antibody binds with the C1 molecule of the complement system and activates this system (one antigen-antibody complex can activate many C1 molecules)
 * The C1 molecule (enzyme) activates increasing numbers of other enzymes in the system (amplification)
 * Multiple end-products have different effects on the pathogens e.g: opsonisation followed by phagocytosis, lysis, agglutination, neutralisation of viruses, chemotaxis, activation of mast cells and basophils and inflammatory effects