CD19 is a 95 kD transmembrane glycoprotein that belongs to the immunoglobulin superfamily. It is a type I transmembrane protein with a single transmembrane domain, a cytoplasmic C-terminus, and an extracellular N-terminus. CD19 is crucial for B cell development and signaling as it modulates both B cell receptor (BCR)-dependent and independent signaling pathways. It is involved in the humoral immune response and tolerance induction, functioning as a critical co-receptor that amplifies the activation of Src-family protein tyrosine kinases.