CD19 is a transmembrane glycoprotein and a member of the immunoglobulin superfamily, predominantly expressed on B lymphocytes, making it a significant marker for B cell development and function. It plays a crucial role in B cell activation, signal transduction, and growth regulation. CD19 is involved in the B cell receptor (BCR) signaling pathway, enhancing B cell activation thresholds and proliferation. In the context of disease, CD19 is highly expressed in most B cell malignancies, including acute lymphoblastic leukemia (ALL), chronic lymphocytic leukemia (CLL), and B cell lymphomas, making it a promising target for immunotherapy. The CD19 molecule is also implicated in autoimmune and immunodeficiency disorders, with mutations in CD19 associated with severe immunodeficiency syndromes characterized by reduced antibody production.