The HIB19 monoclonal antibody reacts with a human 95 kDa transmembrane glycoprotein known as CD19, which is expressed by B lymphocytes during all the developmental stages, except for the terminally differentiated plasma cells. CD19 is also expressed on follicular dendritic cells, and seems to ensure the regulation of B lymphocytes proliferation. CD19, CD21, CD81, MHC class II, and Leu13 can bind together and form a complex which associates with the B cell receptor (BCR) on the surface of B lymphocytes and facilitates the signal transduction.