CD19

CD19 is a transmembrane glycoprotein of the immunoglobulin superfamily that serves as a B cell–specific cell surface marker. The CD19 protein is encoded by the CD19 gene and is expressed on B lineage cells from early development through mature B cells, but is not present on T cells or most plasma cells.

Functionally, CD19 acts as a signaling co-receptor for the B cell receptor (BCR) complex. By amplifying BCR-mediated signaling, it lowers the threshold for B cell activation and supports proliferation, survival, and differentiation. CD19 interacts with other surface molecules such as CD21 and CD81 as part of a co-receptor network that enhances B cell responses and helps integrate extracellular cues with intracellular signaling pathways, including the PI3K pathway.

Expression patterns of CD19 are used in both research and clinical settings. It is broadly expressed on naive and memory B cells and is commonly absent on plasma cells, making it a useful marker for identifying B cell lineages. Loss or alteration of CD19 expression can occur in certain disease states or after targeted therapies, which has diagnostic and monitoring implications.

Clinical significance includes its role as a key immunophenotypic marker in flow cytometry for identifying B cells and B cell malignancies, such as B-acute lymphoblastic leukemia and various B-cell lymphomas. CD19 has become a major therapeutic target in B-cell cancers; therapies include monoclonal antibodies, antibody-drug conjugates, bispecific T cell engagers, and CD19-directed chimeric antigen receptor (CAR) T cells. Examples include blinatumomab and CAR-T products like tisagenlecleucel and axicabtagene ciloleucel, along with newer anti-CD19 agents.