B-lymphocyte antigen CD19 also known as CD19 (Cluster of Differentiation 19), is a protein that in humans is encoded by the CD19gene. It is found on the surface of B-cells, a type of white blood cell.
Lymphocytes proliferate and differentiate in response to various concentrations of different antigens. The ability of the B cell to respond in a specific, yet sensitive manner to the various antigens is achieved with the use of low-affinity antigen receptors. The CD19 gene encodes a cell surface molecule that assembles with the antigen receptor of B lymphocytes in order to decrease the threshold for antigen receptor-dependent stimulation.
CD19 is expressed on follicular dendritic cells and B cells. In fact, it is present on B cells from earliest recognizable B-lineage cells during development to B-cell blasts but is lost on maturation to plasma cells. It primarily acts as a B cell co-receptor in conjunction with CD21 and CD81. Upon activation, the cytoplasmic tail of CD19 becomes phosphorylated, which leads to binding by Src-family kinases and recruitment of PI-3 kinase.
As on T cells, several surface molecules form the antigen receptor and form a complex on B lymphocytes. The (almost) B cell-specific CD19 phosphoglycoprotein is one of these molecules. The others are CD21 and CD81. These surface immunoglobulin (sIg)-associated molecules facilitate signal transduction. On living B cells, anti-immunoglobulin antibody mimicking exogenous antigen causes CD19 to bind to sIg and internalize with it. The reverse process has not been demonstrated, suggesting that formation of this receptor complex is antigen-induced. This molecular association has been confirmed by chemical studies.
Mutations in CD19 are associated with severe immunodeficiency syndromes characterized by diminished antibody production.
Since CD19 is a hallmark of B-cells, the protein has been used to diagnose cancers that arise from this type of cell - notably B-cell lymphomas. Since 2011 treatments targeting CD19 have begun to enter trials. Most current experimental anti-CD19 drugs in development work by exploiting the presence of CD19 to direct treatment specifically towards B-cell cancers. However, it is now emerging that the protein plays an active role in driving the growth of these cancers, most intriguingly by stabilising the concentrations of the MYC oncoprotein. This suggests that CD19 and its downstream signalling may be a more attractive therapeutic target than suspected 
CD19 has also been implicated in autoimmune diseases and may be a useful treatment target.
^Tedder TF, Isaacs CM (July 1989). "Isolation of cDNAs encoding the CD19 antigen of human and mouse B lymphocytes. A new member of the immunoglobulin superfamily". J. Immunol.143 (2): 712–7. PMID2472450.
^ abBradbury LE, Kansas GS, Levy S, Evans RL, Tedder TF (November 1992). "The CD19/CD21 signal transducing complex of human B lymphocytes includes the target of antiproliferative antibody-1 and Leu-13 molecules". J. Immunol.149 (9): 2841–50. PMID1383329.
^ abHorváth G, Serru V, Clay D, Billard M, Boucheix C, Rubinstein E (November 1998). "CD19 is linked to the integrin-associated tetraspans CD9, CD81, and CD82". J. Biol. Chem.273 (46): 30537–43. doi:10.1074/jbc.273.46.30537. PMID9804823.
^ abImai T, Kakizaki M, Nishimura M, Yoshie O (August 1995). "Molecular analyses of the association of CD4 with two members of the transmembrane 4 superfamily, CD81 and CD82". J. Immunol.155 (3): 1229–39. PMID7636191.
^van Zelm MC, Reisli I, van der Burg M, Castaño D, van Noesel CJ, van Tol MJ, Woellner C, Grimbacher B, Patiño PJ, van Dongen JJ, Franco JL (May 2006). "An antibody-deficiency syndrome due to mutations in the CD19 gene". N. Engl. J. Med.354 (18): 1901–12. doi:10.1056/NEJMoa051568. PMID16672701.
^Fujimoto, M.; Sato, S. (2007). "B cell signaling and autoimmune diseases: CD19/CD22 loop as a B cell signaling device to regulate the balance of autoimmunity". Journal of Dermatological Science46 (1): 1–9. doi:10.1016/j.jdermsci.2006.12.004. PMID17223015. edit
Goldsby, Richard A.; Kindt, Thomas J.; Osborne, Barbara A. (2006). Kuby Immunology. San Francisco: W. H. Freeman. ISBN0-7167-8590-0.
Ishikawa H, Tsuyama N, Mahmoud MS, et al. (2003). "CD19 expression and growth inhibition of tumours in human multiple myeloma.". Leuk. Lymphoma43 (3): 613–6. doi:10.1080/10428190290012146. PMID12002767.
Zhou LJ, Ord DC, Omori SA, Tedder TF (1992). "Structure of the genes encoding the CD19 antigen of human and mouse B lymphocytes.". Immunogenetics35 (2): 102–11. doi:10.1007/BF00189519. PMID1370948.
Bradbury LE, Kansas GS, Levy S, et al. (1992). "The CD19/CD21 signal transducing complex of human B lymphocytes includes the target of antiproliferative antibody-1 and Leu-13 molecules.". J. Immunol.149 (9): 2841–50. PMID1383329.
Zhou LJ, Ord DC, Hughes AL, Tedder TF (1991). "Structure and domain organization of the CD19 antigen of human, mouse, and guinea pig B lymphocytes. Conservation of the extensive cytoplasmic domain.". J. Immunol.147 (4): 1424–32. PMID1714482.
Ord DC, Edelhoff S, Dushkin H, et al. (1994). "CD19 maps to a region of conservation between human chromosome 16 and mouse chromosome 7.". Immunogenetics39 (5): 322–8. doi:10.1007/BF00189228. PMID7513297.
Weng WK, Jarvis L, LeBien TW (1995). "Signaling through CD19 activates Vav/mitogen-activated protein kinase pathway and induces formation of a CD19/Vav/phosphatidylinositol 3-kinase complex in human B cell precursors.". J. Biol. Chem.269 (51): 32514–21. PMID7528218.
Chalupny NJ, Aruffo A, Esselstyn JM, et al. (1995). "Specific binding of Fyn and phosphatidylinositol 3-kinase to the B cell surface glycoprotein CD19 through their src homology 2 domains.". Eur. J. Immunol.25 (10): 2978–84. doi:10.1002/eji.1830251040. PMID7589101.
Tuscano JM, Engel P, Tedder TF, et al. (1996). "Involvement of p72syk kinase, p53/56lyn kinase and phosphatidyl inositol-3 kinase in signal transduction via the human B lymphocyte antigen CD22.". Eur. J. Immunol.26 (6): 1246–52. doi:10.1002/eji.1830260610. PMID8647200.
Carter RH, Doody GM, Bolen JB, Fearon DT (1997). "Membrane IgM-induced tyrosine phosphorylation of CD19 requires a CD19 domain that mediates association with components of the B cell antigen receptor complex.". J. Immunol.158 (7): 3062–9. PMID9120258.
Husson H, Mograbi B, Schmid-Antomarchi H, et al. (1997). "CSF-1 stimulation induces the formation of a multiprotein complex including CSF-1 receptor, c-Cbl, PI 3-kinase, Crk-II and Grb2.". Oncogene14 (19): 2331–8. doi:10.1038/sj.onc.1201074. PMID9178909.
Thunberg U, Gidlöf C, Bånghagen M, et al. (1998). "HpaII polymerase chain reaction restriction fragment length polymorphism in the human CD19 gene on 16p11.". Hum. Hered.48 (4): 230–1. doi:10.1159/000022806. PMID9694255.
Horváth G, Serru V, Clay D, et al. (1998). "CD19 is linked to the integrin-associated tetraspans CD9, CD81, and CD82.". J. Biol. Chem.273 (46): 30537–43. doi:10.1074/jbc.273.46.30537. PMID9804823.
Buhl AM, Cambier JC (1999). "Phosphorylation of CD19 Y484 and Y515, and linked activation of phosphatidylinositol 3-kinase, are required for B cell antigen receptor-mediated activation of Bruton's tyrosine kinase.". J. Immunol.162 (8): 4438–46. PMID10201980.