CD19

B-lymphocyte antigen CD19, also known as CD19 molecule (Cluster of Differentiation 19), B-Lymphocyte Surface Antigen B4, T-Cell Surface Antigen Leu-12 and CVID3 is a transmembrane protein that in humans is encoded by the gene CD19. In humans, CD19 is expressed in all B lineage cells. Contrary to some early doubts, human plasma cells do express CD19, as confirmed by others. CD19 plays two major roles in human B cells: on the one hand, it acts as an adaptor protein to recruit cytoplasmic signaling proteins to the membrane; on the other, it works within the CD19/CD21 complex to decrease the threshold for B cell receptor signaling pathways. Due to its presence on all B cells, it is a biomarker for B lymphocyte development, lymphoma diagnosis and can be utilized as a target for leukemia immunotherapies. In humans, CD19 is encoded by the 7.41 kilobase CD19 gene located on the short arm of chromosome 16. It contains at least fifteen exons, four that encode extracellular domain and nine that encode cytoplasmic domains, with a total of 556 amino acids. Experiments show that there are multiple mRNA transcripts; however, only two have been isolated in vivo. CD19 is a 95 kd Type I transmembrane glycoprotein in the immunoglobulin superfamily (IgSF) with two extracellular C2-set Ig-like domains and a relatively large, 240 amino acid, cytoplasmic tail that is highly conserved among mammalian species. The extracellular C2-type Ig-like domains are divided by a potential disulfide linked non-Ig-like domain and N-linked carbohydrate addition sites. The cytoplasmic tail contains at least nine tyrosine residues near the C-terminus. Within these residues, Y391, Y482, and Y513 have been shown to be essential to the biological functions of CD19. Phenylalanine substitution for tyrosine at Y482 and Y513 leads to the inhibition of phosphorylation at the other tyrosines. CD19 is widely expressed during all phases of B cell development until terminal differentiation into plasma cells.