CD19 is a 95kDa transmembrane glycoprotein that belongs to the immunoglobulin superfamily. It is expressed in various stages of B cell development, except for plasma cells. CD19 forms a complex with CD21, CD81, and MHC II molecules and plays a role in the activation of immature B cells through interaction with the B cell receptor. CD19 promotes B cell proliferation, development, and activation, as well as the maturation of memory B cells. In mice lacking CD19, both bone marrow and peripheral B cell generation and maturation are impaired.