The HB7 monoclonal antibody reacts with the human CD38 molecule, a type II transmembrane protein. Expression of CD38 is bimodal during B cell development, modulating from high in immature cells to low in intermediate ones and back to high on mature B cells. Additionally CD38 is found in a variety of tissues and other hematopoietic cell. The CD38 antigen acts as a bifunctional ectoenzyme that catalyzes both the synthesis and the hydrolysis of a Ca++ mobilizing agent, cyclic ADP-ribose. This intracellular calcium plays an important role in cell signaling pathways.