Antibody classes and their constituent polypeptides

Antibodies are multichain proteins with an equal number of heavy chains (50-70 kDa) and light chains (about 25 kDa). Most Ig possess only the four-chain basic unit consisting of two identical light chain-heavy chain pairs linked by disulfide bridges and non-covalent interactions. The basic units are symmetrical, i.e. each half contains one light and one heavy chain, linked together by a disulfide bond, resulting in a Y-like structure. For some antibody classes (IgA and IgM), polymers of Ig with an extra chain (J chain) are encountered, whereas IgA in external secretions carries an additional polypeptide, the secretory component (SC). 

Antibodies have a dual function (i) recognition of the antigen and, (ii) the activation, after antigen recognition, of a variety of 

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