A small white blood cell that plays a major role in defending the body against disease. There are two main types of lymphocytes: B cells, which make antibodies that attack bacteria and toxins, and T cells, which attack body cells themselves when they have been taken over by viruses or become cancerous.

Dr. Jennifer Puck, of the National Human Genome Research Institute's Genetics and Molecular Biology Branch, defines lymphocyte.

Related terms: cancer, cell.