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

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

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

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

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

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

HALL, John E. Guyton and Hall Textbook of Medical Physiology. 11th Edition edition. 2006. ISBN 978-1-4160-4574-8.