CD19 is a transmembrane protein found in B cells other than plasma and follicular dendritic cells. CD19 is a necessary adaptor protein in signal transduction pathways that regulate B-cell differentiation and development. Additionally, mutations in CD19 have been associated with a decrease in antibody production in immunodeficiency syndromes and have also been implicated in autoimmune diseases. CD19 is a common marker for B-cells – it demonstrates positive membranous staining in B-cells in the mantle zone, germinal centers of lymphoid tissues, and in occasional plasma cells. In cancers, it is positive in B-cell lymphomas, most B-cell leukemia and in hairy cell leukemia. It is negative in T-cell lymphomas. References: Adv Exp Med Biol. 2005;560:125-39, PMID: 15934172; The Journal of Experimental Medicine. 1989 Dec. 170 (6): 2159–64, PMID: 2479707; The New England Journal of Medicine. 2006 May. 354 (18): 1901–12. PMID: 16672701; Leukemia & Lymphoma. 1995 Aug. 18 (5–6): 385–97, PMID: 8528044.