HLA class II DR beta chain proteins (26-28 kD) form heterodimers with alpha (DRA) chains, both anchored in the membrane. Expressed in antigen presenting cells (B lymphocytes, dendritic cells, macrophages), alpha/beta heterodimers present peptides derived from extracellular proteins. Hundreds of DRB alleles have been described and typing for these polymorphisms is done for transplantation. (NCI) [ ]