Lymphocytes are small white blood cells that bear the major responsibility for carrying out the activities of the immune system; they number about one trillion. The two major classes of lymphocytes are: B cells, which grow to maturity independent of the thymus, and T cells, which are processed in the thymus. Both B cells and T cells recognize specific antigen targets.

B cells work chiefly by secreting soluble substances called antibodies into the body's fluids, or humors. (This is known as humoral immunity.) Antibodies typically interact with circulating antigens such as bacteria and toxic molecules, but are unable to penetrate living cells. T cells, in contrast, interact directly with their targets, attacking body cells that have been commandeered by viruses or warped malignancy. (This is cellular immunity.)



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Note added at 2003-05-18 18:00:58 (GMT)
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Lymphocyte
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.

http://www.hon.ch/Library/Theme/Allergy/Glossary/lymphocyte....

The main lymphocyte sub-types are :

B-Cells . Special B cells produce specific antibodies, proteins that help destroy foreign substances.
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.
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.
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 .

B and T-lymphocytes of the immune system