CD79 is a 23/21 disulfide-linked heterodimer composed of an alpha chain (CD79a/mb-1) and a beta chain (CD79b/B29) that associates non-covalently with membrane immunoglobulin to form the B cell receptor (BCR) complex. Its expression is restricted to B lymphocytes, first appearing on the surface at the pro-B cell stage and remaining through all stages of B cell differentiation prior to plasma cells. Crosslinking of the BCR leads to B cell activation.