The human CD19 antigen is a 95 kDa transmembrane glycoprotein belonging to the immunoglobulin (Ig) superfamily.  It is also known as T-Cell Surface Antigen Leu-12 or B-Lymphocyte Surface Antigen B4. CD19 is present on B cells from development to B-cell blasts, but is lost to plasma cells on maturation. As one of the most reliable B cell biomarkers, CD19 has been used in the diagnosis of cancers that arise from this type of cell. This includes acute lymphoblastic leukemia (ALL), B cell lymphomas, and chronic lymphocytic leukemia (CLL).