The class II molecules of the human major histocompatibility complex are cell surface glycoproteins encoded by a series of genes located on the short arm of chromosome 6 in the HLA-D region. These molecules consist of an α and � chain that associate as heterodimers on the surface of antigen presenting cells. They serve as receptors for processed peptides derived primarily from mem-brane and extracellular proteins that they present to CD4+ lymphocytes.