CD19 is a 95 kDa type-I transmembrane glycoprotein which is restricted to B cell antigen. CD19 antigen is expressed on normal and neoplastic B cells and also in some bone marrow cells. CD19 expression by B progenitor cells is presumably at late pro-B or early pre-B stages around the time of Ig heavy chain rearrangement. Expression persists during all stages of B cell maturation and is lost on terminal differentiation to plasma cells. CD19 antigen is also found on the follicular dendritic cells and the early cells of myelomonocytic lineage but not on normal T cells, NK cells, monocytes, granulocytes, erythrocytes and platelets. In normal peripheral blood, 8-20% of lymphocytes express CD19 antigen. CD19 antigen plays a role in regulating B cell proliferation.