CD19 is a B-cell-restricted signaling molecule. It modulates both BCR-dependent and BCR-independent signaling, thereby establishing inherent B cell signaling thresholds. Lack of CD19 results in faulty BCR (B cell receptor) signaling. CD19 plays an essential role in B cell development and maturation, antigen-independent development, and immunoglobulin-induced activation of B cells. The body also needs CD19 to mount an optimal immune response.