CD19 

CD19 is a 95 kD transmembrane glycoprotein that is highly expressed in human B cells and certain types of tumors, belonging to the immunoglobulin superfamily. CD19 plays a crucial role in regulating the threshold of B cell signaling by modulating both B cell receptor (BCR)-dependent and independent signaling pathways, thereby playing an essential role in the initiation of immune responses and the maintenance of tolerance. CD19 typically forms a multimolecular complex with molecules like CD21 and CD81 on the surface of mature B cells, where it exerts its dominant signaling function.
CD19 works in cooperation with the BCR to promote the activation of Src family protein tyrosine kinases, enhancing B cell proliferation and expansion. It can also independently enhance B cell activation by binding to the complement receptor CD21. Mice and humans lacking CD19 show weakened T cell-dependent immune responses and low levels of immunoglobulins, leading to immune dysfunction. High expression of CD19 is closely associated with B cell lymphoma, acute lymphoblastic leukemia (ALL), and other malignancies. CD19 monoclonal antibodies are used in the study of B cell malignancies^[1].