Blymphocyte antigen CD19 is also known as CD19 (Cluster of Differentiation 19), is a singlepass type I membrane protein which contains two Iglike C2type (immunoglobulinlike) domains. CD19 is expressed on follicular dendritic cells and B cells. In fact, it is present on B cells from earliest recognizable Blineage cells during development to Bcell blasts but is lost on maturation to plasma cells. It primarily acts as a B cell coreceptor in conjunction with CD21 and CD81. Upon activation, the cytoplasmic tail of CD19 becomes phosphorylated, which leads to binding by Srcfamily kinases and recruitment of PI3 kinase. As on T cells, several surface molecules form the antigen receptor and form a complex on B lymphocytes. The (almost) B cellspecific 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, antiimmunoglobulin 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 antigeninduced. This molecular association has been confirmed by chemical studies. Mutations in CD19 are associated with severe immunodeficiency syndromes characterized by diminished antibody production. CD19 has been shown to interact with: CD81, CD82, Complement receptor 2, and VAV2. - Proteine/Peptide
