Structure, function, and evolutionary relationships of Fc domains of human immunoglobulins A, G, M, and E.

Human immunoglobulins, A, G, M, and E have strong homology in amino acid sequence (about 30 percent) distributed nonuniformly throughout the Fc region. Immunoglobulins M are A are least alike in the first Fc domain and most alike in the second. Individual domains of heavy chains have evolved with different mutation rates but with conservation of essential structural features. No relation of primary structure to complement binding ability is apparent.