CD19 is a 95 kDa type I transmembrane glycoprotein that belongs to the immunoglobulin superfamily and is expressed on the surface of B lymphocytes and follicular dendritic cells. It plays a crucial role in B cell activation and humoral immune responses by forming a complex with CD21, CD81, and CD225. This complex reduces the threshold for B cell receptor (BCR) activation, enhancing downstream signaling pathways and facilitating B cell responses to antigens. CD19 is also a biomarker for B cell development and lymphoma diagnosis, and it is targeted in immunotherapies for diseases like leukemia.