B-lymphocyte antigen CD19 also known as CD19 (C luster of D ifferentiation 19), is a protein that in humans is encoded by the CD19 gene.It is found on the surface of B-cells, a type of white blood cell.s on T cells, several surface molecules form the antigen receptor and form a complex on B lymphocytes. The (almost) B cell-specific CD19 phosphoglycoprotein is one of these molecules. The others are CD21 and CD81.