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.