Large, transmembrane, non-covalently linked Glycoproteins (alpha and beta). Both chains can be polymorphic although there is more structural variation in the beta chains. The class II Antigens in Humans are called HLA-D Antigens and are coded by A Gene on Chromosome 6. In Mice, two Genes named IA and IE on Chromosome 17 code for the H-2 Antigens. The Antigens are found on B-Lymphocytes, Macrophages, epidermal Cells, and Sperm and are thought to mediate the competence of and cellular cooperation in the immune response. The term IA Antigens used to refer only to the Proteins encoded by the IA Genes in the Mouse, but is now used as A Generic term for any class II Histocompatibility Antigen.