White blood cell (leukocyte) containing no granules that normally makes up about 25% of the total white blood cell count but increases in the presence of infection. Lymphocytes occur in two main forms:
- B cells, the chief agents of the humoral immune system, which recognize specific antigens and produce antibodies against them; and
- T cells, the agents of the cell-mediated immune system, which secrete immunologically active compounds and assist B cells in their function.