Antibodies

Antibodies are immunoglobulin (Ig) proteins synthesised in response to antigen exposure, and each one binds specifically to the antigen that provoked its production. Whenever a pathogen enters the body, B lymphocytes differentiate into plasma cells, which secrete an army of these antibody proteins to neutralise it. Based on their physiological and biochemical properties, antibodies fall into five major classes: IgG (gamma), IgM (mu), IgA (alpha), IgD (delta) and IgE (epsilon).

Structurally, an antibody monomer is a Y-shaped molecule built from four polypeptide chains: two identical light (L) chains (about 25,000 Da, roughly 214 amino acids each) and two identical, larger heavy (H) chains (about 50,000 Da, roughly 450 amino acids each), all held together by disulphide (S–S) bonds — one light chain pairs with each heavy chain, and the two heavy chains join each other, giving the whole molecule the formula H₂L₂. The heavy chains carry a flexible hinge region roughly at their midpoint, which is what lets the two 'arms' of the Y splay apart to grip antigen at different angles. …