Abstract
The complete amino acid sequence of the .alpha.2 H chain of a human Ig (immunoglobulin) A2 of the A2m(2) allotype was determined and was compared to the sequence of the .alpha.1 chain of the human IgA1 subclass. The characteristic differences between the .alpha.1 and .alpha.2 chains are greatest in the hinge region and in the location and number of the oligosaccharides. Apart from the duplication in the hinge region of .alpha.1 and the deletion in .alpha.2, there are 23 amino exchanges in the constant (C) regions of the 2 chains. Accepted mutations are related to the surface accessibility of the residues and the proximity of carbohydrate. Human IgA and IgG subclasses apparently arose late in evolution and reflect similar mutational pressures.