Studies involving the cleavage of the immunoglobulin molecule, pioneered by Porter, Edelman, Nisonoff and their colleagues, have led to a detailed picture of its structure. Rabbit IgG antibody to egg albumin, digested by papain in the presence of cysteine, was split into two fractions an insoluble fraction which crystallised in the cold (called Fc for crystallized), and a soluble fragment which, while unable to precipitate with egg albumin, could still bind with it. This fragment is called

the Fab (antigen binding) fragment. Each molecule of immunoglobulin is split by papain into three parts one Fc and two Fab pieces, having a sedimentation coefficient of 3.5 S. When treated with pepsin, a 5 S fragment is obtained, which is composed essentially of two Fab fragments held together in position. It is

bivalent and precipitates with the antigen. This fragment is called F(ab')2. The Fc portion is digested by pepsin into smaller fragments Immunoglobulins are glycoproteins, each moleculeconsisting of two pairs of polypeptide chains of different sizes. The smaller chains are called 'light' (L) chains and the larger ones 'heavy' (H) chains.

The Lichen has a molecular weight of approximately 25,000 and the H chain of 59,000. The L chain is attached to the H chain by a disulfide bond. The two H chains are joined together by 1-5 S-S bonds,

depending on the class of immunoglobulins.

The H chains are structurally and antigenically

distinct for each class and are designated by the Greekletter corresponding to the immunoglobulin class.