CD19 is a member of the immunoglobulin gene superfamily. CD19 plays a significant role in B-cell development and function, as CD19 is selectively expressed on the surface of B-lymphocytes. CD19 contains two extracellular immunoglobulin-like domains, which are essential for interaction with other surface proteins and for initiating intracellular signaling pathways. Notably, CD19 forms functional complexes with proteins such as integrin β1, CD21, and CD81, which are vital for regulating B-cell activation, proliferation, and differentiation. Activation of CD19 leads to cross-linking of immunoglobulin chains and recruitment of Src family protein tyrosine kinases, thereby triggering downstream signaling cascades that are critical for B-cell responses.