B-cell development begins in the bone marrow. Maturation of these progenitor cells coincides with the expression of specific surface receptors and antigens, one of which is a surface antigen called CD19. Under normal circumstances, this antigen exists exclusively in B-cells. CD19 is a glycoprotein that consists of an extracellular n-terminus with 2 domains, a transmembrane domain, and a cytoplasmic c-terminus. CD19 is a component of the primary signaling complex on the surface of B-cells. It is a part of pathways that function to modulate B-cell involvement in the immune response. When the B-cell binds to an antigen, CD19 plays a role in lowering the triggering threshold for the antigen-initiated pathway. Overexpression of CD19 is characteristic in many B-cell and plasma cell malignancies, making it a target for therapies.
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