The CD19 antigen plays an important role in clinical oncology. It’s a protein found on the surface of B-cells, a type of white blood cell. Since CD19 is a hallmark of B-cells, the protein has been used to diagnose cancers that arise from this type of cell - notably B-cell lymphomas. CD19 has also been implicated in autoimmune diseases and may be a useful treatment target. CD19 is also expressed in a subset of acute myelogenous leukemias indicating the close relationship between the lymphoid and myeloid lineages.