CD19 antigen, also known as B Lymphocyte Antigen CD19 or Membrane Antigen B4. CD19 is a transmembrane glycoprotein belonging to the immunoglobulin superfamily and is one of the most important markers on the B cell surface. CD19 is consistently expressed at high levels throughout B cell development (from pre-B cell to mature B cell) and is also highly expressed in most B cell malignancies, such as B cell lymphomas and leukemias. Functionally, CD19 is a critical component of the B cell receptor (BCR) signaling complex, regulating B cell activation, proliferation, and differentiation. The high and consistent expression of CD19 on malignant cells and its limited distribution in normal tissues make it an ideal target for selectively eliminating cancerous B cells.