A mouse-derived anti-human CD19 monoclonal antibody linked to pokeweed (Phytolacca americana) antiviral protein (PAP) with antileukemic activity. The monoclonal antibody portion specifically binds to the CD19 antigen, a cell surface molecule normally expressed only by B lymphocytes and follicular dendritic cells and over-expressed in B-lineage lymphocytic leukemia cells. Following internalization, PAP, a plant hemitoxin and a ribosome-inactivating protein, is cleaved from the immunoconjugate and released into the cytoplasm where it enzymatically removes a single adenine base from a conserved, surface exposed loop sequence of rRNA leading to inhibition of protein synthesis and cell growth, but not necessarily cell death. or A type of white blood cell that makes antibodies. B lymphocytes are part of the immune system and develop from stem cells in the bone marrow. Also called B cell.