A white blood cell of the mononuclear leukocyte subgroup (like macrophage / monocytes ). Lymphocytes identify foreign substances and germs (bacteria or viruses) in the body and produce antibodies and cells that specifically target them. It takes from several days to weeks for lymphocytes to recognize and attack a new foreign substance.

The main lymphocyte sub-types are :

1. B-Cells . Special B cells produce specific antibodies, proteins that help destroy foreign substances.
2. T-Cells . T-cells attack virus-infected cells, foreign tissue, and cancer cells. They also produce a number of substances that regulate the immune response.
3. NK Cells . Among other functions, natural killer cells destroy cancer cells and virus-infected cells through phagocytosis and by producing substances that can kill such cells.
4. Null cells . An early population of lymphocytes bearing neither T-cell nor B-cell differentiation antigens.

Lymphocytes are small cells with virtually no cytoplasm, found in the blood, in all tissue and in lymphoid organs, such as lymph nodes , the tonsils , thymus gland , spleen and Peyer's patches .