Figure 1: (A) MHC (major histocompatibility complex). (B) Class II antigens are expressed only on B lymphocytes, activated T lymphocytes, monocytes, macrophages, Langerhans cells, dendritic cells, endothelium, and epithelial cells. They are heterodimers composed of noncovalently associated α and β polypeptide chains chains encoded by genes of the HLA-D region. (C) Class I MHC antigens are present on all nucleated cells and are composed of a 45-kd transmembrane α heavy chain encoded by genes of the HLA-A, HLA-B, or HLA-C loci on chromosome 6.