HLA DR, like other MHC class II molecules, is a transmembrane glycoprotein composed of an alpha chain (36 kDa) and a beta chain (27 kDa). It is expressed primarily on antigen presenting cells such as B lymphocytes, monocytes, macrophages, thymic epithelial cells and activated T lymphocytes. Three loci, DR, DQ and DP, encode the major expressed products of the human class II region. The human MHC class II molecules bind intracellularly processed peptides and present them to T helper cells. They therefore have a critical role in the initiation of the immune response.