Immunoglobulin G (IgG). Pairs of heavy and light chains combine to form a Y-shaped molecule. Two antigen-binding sites are formed by the combination of variable domains from one light and one heavy chain. The light chains include V (variable) and J (joining) segments. The heavy chains have V (variable), D (diverse), and J (joining) segments, brought together by mechanisms similar to those for light chains, as described in the text. For heavy chains in the human genome, there are 44 V segments, 27 D segments, and 6 J segments, with one of each brought together at random in a given immunoglobulin.