The CD19 antigen (also called B4) is a type I membrane glycoprotein with a molecular weight of 95 kDa. CD19 is involved in the regulation of B lymphocyte development, activation and differentiation. This molecule is expressed on all normal B lymphocytes including pro-B lymphocytes. It is also found on the surface of follicular dendritic cells, on the early cells of myelomonocytic lineage and on most stabilized B cell lines. It is not present on normal T lymphocytes, NK cells, monocytes, and granulocytes.