CD19 is a transmembrane glycoprotein that contains two extracellular immunoglobulin- like domains. CD19 is selectively expressed on the cell surface of B lymphocytes, where it activates intracellular signaling cascades involving both Ras and phosphatidylinositol 3-kinase pathways. Activation of CD19 results in cross-linking of the membrane protein immunoglobulin chains and the subsequent association with Src family protein tyrosine kinases (PTK). Expression of CD19 is continuous throughout B cell development and through terminal differentiation of B cells into plasma cells. CD19 forms functional complexes with B lymphocyte surface proteins, including integrin β1, CD21 and CD81, which are involved in regulating B cell development.Synonyms: B-cell marker, B-lymphocyte surface antigen B4, Differentiation antigen CD19, Leu-12