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Lymphocyte (stained)
A scanning electron microscope (SEM) image of a single human lymphocyte.

Lymphocytes are a type of white blood cell (or leukocyte). They help an organism to fight infections. They occur in the immune system of all vertebrates.[1] All lymphoctes have a large, blob-like nucleus.

Lymphocytes can be divided into three main types:

Types[change | change source]

Natural killer cells[change | change source]

Natural killer (NK) cells are a part of the immune system a person is born with. They protect the organism from tumours and from cells infected by viruses. Natural killer cells are made active by chemicals released by damaged cells called interferons.

Natural killer cells can tell the difference between infected cells, tumours, and normal cells. They recognise changes in the level of a surface molecule called MHC (major histocompatibility complex) class I.

The NK cells then release cytotoxic (cell-killing) granules which then destroy the altered cells.[2]

T and B cells[change | change source]

T cells (thymus cells) and B cells (bone cells) are the main cells of the adaptive immune response. They tackle infections, and they cause the immune system to 'remember' the event.

The function of T cells and B cells is to recognize foreign antigens. Antigens are surface molecules on a cell. Once they have identified an invader, the cells respond to remove pathogens or pathogen-infected cells.

B cells respond to pathogens by producing large numbers of antibodies which then destroy foreign objects like bacteria and viruses.

Some T cells, called T helper cells, produce cytokines that direct the immune response. Cytokines signal to other immune cells that there is a foreign antigen present. Other T cells, called cytotoxic T cells, produce toxic granules which cause the death of infected cells.

Once they are made active, B cells and T cells produce memory cells. Throughout the lifetime of an animal, these cells will 'remember' each specific pathogen encountered, and are able to make a strong response if the pathogen is detected again. Memory cells live a long time in the body, and are how vaccinations work.

Development[change | change source]

Mammalian stem cells change or differentiate into several kinds of blood cell within the bone marrow.[3] This process is called haematopoiesis (= blood growth). All lymphocytes come from a common basic lymphocyte cell before differentiating into their distinct lymphocyte types.

B cells mature into B lymphocytes in the bone marrow,[4] while T cells migrate to and mature in a distinct organ, called the thymus. After they mature, the lymphocytes enter the circulation and lymphoid organs (e.g. the spleen and lymph nodes) where they are able to sense invading pathogens and tumour cells.

Characteristics[change | change source]

Function and proportion of lymphocytes [5]
Class Function Proportion
Natural killer cells Lysis of virally infected cells and tumour cells 7% (2-13%)
Helper T cells Release cytokines and growth factors that regulate other immune cells 46% (28-59%)
Cytotoxic T cells Lysis of virally infected cells, tumour cells and allografts 19% (13-32%)
gamma/delta T cells Immunoregulation and cytotoxicity
B cells Secretion of antibodies 23% (18-47%)

In the circulatory system they move from lymph node to lymph node. This contrasts with macrophages, which are rather stationary in the nodes.

Lymphocytes and disease[change | change source]

When white blood cells are counted, the lymphocyte count is the percentage of lymphocytes present.

An increase in lymphocyte concentration is usually a sign of a viral infection (in some rare case, leukemias are found through an abnormally high lymphocyte count in an otherwise normal person).

A low normal to low absolute lymphocyte count is found with infections after surgery or trauma.

Human immunodeficiency virus (HIV) infects and destroys T cells.[6] Without the key defence that these T cells provide, the body becomes prone to infections that otherwise would not affect healthy people.

The extent of HIV progression is found by measuring the percentage of CD4+ T cells in the patient's blood. The effects of other virus or lymphocyte disorders can also often be estimated by counting the numbers of lymphocytes present in the blood.

References[change | change source]

  1. "Dorlands Medical Dictionary:lymphocyte". Retrieved 2009-01-27.
  2. Janeway, Charles; Paul Travers, Mark Walport, and Mark Shlomchik (2001). Immunobiology; 5th ed. New York and London: Garland Science. ISBN 0-8153-4101-6.{{cite book}}: CS1 maint: multiple names: authors list (link).
  3. Abbas AK and Lichtman AH (2003). Cellular and molecular immunology (5th ed.). Saunders, Philadelphia. ISBN 0-7216-0008-5.
  4. Kumar, Abbas, Fausto. Pathologic basis of disease (7th ed.).{{cite book}}: CS1 maint: multiple names: authors list (link)
  5. Berrington, J. E.; Barge, D; Fenton, AC; Cant, AJ; Spickett, GP (May 2005). "Lymphocyte subsets in term and significantly preterm UK infants in the first year of life analysed by single platform flow cytometry". Clin Exp Immunol. 140 (2): 289–292. doi:10.1111/j.1365-2249.2005.02767.x. PMC 1809375. PMID 15807853.
  6. Specifically, the CD4+ subgroup of T lymphocytes