CD19

B-lymphocyte antigen CD19 also known as CD19 (Cluster of Differentiation 19), is a is_associated_with::protein that in humans is encoded by the CD19 is_associated_with::gene. It is found on the surface of is_associated_with::B-cells, a type of is_associated_with::white blood cell.

Function
is_associated_with::Lymphocytes proliferate and differentiate in response to various concentrations of different is_associated_with::antigens. The ability of the is_associated_with::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 is_associated_with::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 is_associated_with::plasma cells. It primarily acts as a B cell co-receptor in conjunction with is_associated_with::CD21 and is_associated_with::CD81. Upon activation, the cytoplasmic tail of CD19 becomes is_associated_with::phosphorylated, which leads to binding by Src-family kinases and recruitment of is_associated_with::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.

Interactions
CD19 has been shown to interact with:
 * is_associated_with::CD81,
 * CD82
 * is_associated_with::Complement receptor 2, and
 * is_associated_with::VAV2.

In disease
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 is_associated_with::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 stabilizing the concentrations of the is_associated_with::MYC oncoprotein. This suggests that CD19 and its downstream signaling may be a more attractive therapeutic target than suspected

CD19 has also been implicated in autoimmune diseases and may be a useful treatment target.