The Cluster of Differentiation 19 or CD19 molecule is a protein expressed on the cell surface of the B lymphocytes or the B cell. The protein CD19 is a transmembrane protein encoded by the gene CD19. CD19 protein helps in the B cell signaling pathway and recruits cytoplasmic signaling molecules. As CD19 is only expressed in the B Cell, CD19 is the marker for B lymphocytes. Mutation in CD19 leads to severe immunodeficiency syndromes in which the production of antibodies decreased.