CD19 is a transmembrane protein that in humans is encoded by CD19 gene. CD19 is expressed in all B lineage cells, except for plasma cells, and in follicular dendritic cells. CD19 plays two major roles in human B cells. It acts as an adaptor protein to recruit cytoplasmic signaling proteins to the membrane and it works within the CD19/CD21 complex to decrease the threshold for B cell receptor signaling pathways. Due to its presence on all B cells, it is a biomarker for B lymphocyte development, lymphoma diagnosis and can be utilized as a target for leukemia immunotherapies.