Lymphocytes are a type of white blood cell. They are produced in the bone marrow and lymph nodes and stored in the thymus gland, spleen, and lymph nodes.

Lymphocytes play an important role in the body's immune system. Lymphocytes make their way through the lymph channels into the bloodstream, where they identify and memorize the characteristics of foreign elements called antigens. There are two types of lymphocytes -- B cells and T cells. B cells manufacture antibodies (highly specialized proteins that destroy the antigens by combining with them). T cells, which make up 70 percent of the lymphocyte total, regulate antibody production and oversee immune responses.

Lymphocytes are the second most numerous type of white blood cell, normally constituting between 22 and 28 percent of the white blood cells in an adult's circulation. In cases of infection, especially those caused by viruses, the percentage of lymphocytes in the blood may increase to above 50 percent.