CD37 is a 40-64 kDa tetraspanin family glycoprotein, which forms complexes in the B cell membrane with MHC class II, CD53, CD81, and CD82. It is expressed highly on mature B cells and neoplastic B cells, but it is lost on plasma cells, as well as on pro-B cells. Lower expression was detected on monocytes, macrophages, and dendritic cells.