B-lymphocyte antigen CD19, also known as CD19 (Cluster of Differentiation 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. Lymphocytes proliferate and differentiate in response to various concentrations of different antigens. The ability of the B cell to respond in a specific, yet sensitive manner to the various antigens is achieved with the use of low-affinity antigen receptors. The CD19 gene encodes a cell surface molecule that assembles with the antigen receptor of B lymphocytes in order to decrease the threshold for antigen receptor-dependent stimulation.