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07-SEPTEMBER-2008 03:17:44 - Lymphocyte A scanning electron microscope SEM image of a single human lymphocyte. A scanning electron microscope SEM image of a single human lymphocyte. A lymphocyte is a type of white blood cell in the vertebrate immune system. By their appearance under the light microscope, there are two broad categories of lymphocytes, namely the large granular lymphocytes and the small lymphocytes. Functionally distinct subsets of lymphocytes correlate with their appearance. Most, but not all large granular lymphocytes are more commonly known as the natural killer cells NK cells. The small lymphocytes are the T cells and B cells. Lymphocytes play an important and integral role in the body's defenses. Contents 1 Quantity 2 Types of lymphocytes 2.1 Natural killer cells 2.2 T cells and B-cells 3 Lymphocyte development 4 Characteristics 5 Lymphocytes and disease 6 See also 7 References 8 External links Quantity An average human body contains about 1012 lymphoid cells, and the lymphoid tissue as a whole represents about 2% of the total body weight. It is 20-40% 1 of all leucocytes. Normal reference range = 1-5x109 cells per L of blood. Types of lymphocytes A stained lymphocyte surrounded by red blood cells viewed using a light microscope. A stained lymphocyte surrounded by red blood cells viewed using a light microscope. The three major types of lymphocyte are T cells, B cells and natural killer NK cells Natural killer cells Main article: NK cells NK cells are a part of innate immune system and play a major role in defending the host from both tumours and virally infected cells. NK cells distinguish infected cells and tumours from normal and uninfected cells by recognizing alterations in levels of a surface molecule called MHC major histocompatibility complex class I. NK cells are activated in response to a family of cytokines called interferons. Activated NK cells release cytotoxic cell-killing granules which then destroy the altered cells.2 They were named natural killer because of the initial notion that they do not require prior activation in order to kill cells which are missing MHC class I. T cells and B-cells Main articles: T cells and B cells T cells and B-cells are the major cellular components of the adaptive immune response. T cells are involved in cell-mediated immunity whereas B cells are primarily responsible for humoral immunity relating to antibodies. The function of T cells and B cells is to recognize specific non-self antigens, during a process known as antigen presentation. Once they have identified an invader, the cells generate specific responses that are tailored to maximally eliminate specific pathogens or pathogen infected cells. B cells respond to pathogens by producing large quantities of antibodies which then neutralize foreign objects like bacteria and viruses. In response to pathogens some T cells, called helper T cells produce cytokines that direct the immune response while other T cells, called cytotoxic T cells, produce toxic granules that induce the death of pathogen infected cells. Following activation, B cells and T cells leave a lasting legacy of the antigens they have encountered, in the form of memory cells. Throughout the lifetime of an animal these memory cells will remember each specific pathogen encountered, and are able to mount a strong response if the pathogen is detected again. Lymphocyte development Development of blood cells Development of blood cells Mammalian stem cells differentiate into several kinds of blood cell within the bone marrow.3 This process is called haematopoiesis. All lymphocytes originate, during this process, from a common lymphoid progenitor before differentiating into their distinct lymphocyte types. The differentiation of lymphocytes follows various pathways in a hierarchical fashion as well as in a more plastic fashion. The formation of lymphocytes is known as lymphopoiesis. B cells migrate to the spleen and mature into B lymphocytes, while T cells migrate to and mature in a distinct organ, called the thymus. Following maturation, the lymphocytes enter the circulation and peripheral lymphoid organs e.g the spleen and lymph nodes where they survey for invading pathogens and/or tumour cells. The lymphocytes involved in adaptive immunity i.e. B and T cells differentiate further after exposure to an antigen; they form effector and memory lymphocytes. Effector lymphocytes function to eliminate the antigen, either by releasing antibodies in the case of B cells, cytotoxic granules cytotoxic T cells or by signaling to other cells of the immune system helper T cells. Memory cells remain in the peripheral tissues and circulation for an extended time ready to respond to the same antigen upon future exposure. They live weeks to several years1, which is very long compared to other leukocytes. Characteristics A scanning electron microscope image of normal circulating human blood showing red blood cells, several types of white blood cells including lymphocytes, a monocyte, a neutrophil and many small disc-shaped platelets. A scanning electron microscope image of normal circulating human blood showing red blood cells, several types of white blood cells including lymphocytes, a monocyte, a neutrophil and many small disc-shaped platelets. Microscopically, in a Wright's stained peripheral blood smear, a normal lymphocyte has a large, dark-staining nucleus with little to no basophilic cytoplasm. In normal situations, the coarse, dense nucleus of a lymphocyte is approximately the size of a red blood cell about 7 micrometres in diameter.3 Some lymphocytes show a clear perinuclear zone or halo around the nucleus or could exhibit a small clear zone to one side of the nucleus. Polyribosomes are a prominent feature in the lymphocytes and can be viewed with an electron microscope.3 The ribosomes are involved in protein synthesis allowing the generation of large quantities of cytokines and immunoglobulins by these cells. It is impossible to distinguish between T cells and B cells in a peripheral blood smear.3 Normally, flow cytometry testing is used for specific lymphocyte population counts. This can be used to specifically determine the percentage of lymphocytes that contain a particular combination of specific cell surface proteins, such as immunoglobulins or cluster of differentiation CD markers or that produce particular proteins for example, cytokines using intracellular cytokine staining ICCS. In order to study the function of a lymphocyte by virtue of the proteins it generates, other scientific techniques like the ELISPOT or secretion assay techniques can be used.2 Typical recognition markers for lymphocytes4 LYMPHOCYTE CLASS FUNCTION OF LYMPHOCYTE PROPORTION PHENOTYPIC MARKERS NK cells Lysis of virally infected cells and tumour cells 7% 2-13% CD16 CD56 but not CD3 Helper T cells Release cytokines and growth factors that regulate other immune cells 46% 28-59% TCRαβ, CD3 and CD4 Cytotoxic T cells Lysis of virally infected cells, tumour cells and allografts 19% 13-32% TCRαβ, CD3 and CD8 γδ T cells Immunoregulation and cytotoxicity TCRγδ and CD3 B cells Secretion of antibodies 23% 18-47% MHC class II, CD19 and CD21 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 Please help improve this section by expanding it. Further information might be found on the talk page or at requests for expansion. December 2007 A lymphocyte count is usually part of a peripheral complete blood cell count and is expressed as percentage of lymphocytes to total white blood cells counted. An increase in lymphocytes is usually a sign of a viral infection in some rare cases, leukemias are found through an abnormally raised lymphocyte count in an otherwise normal person. A general increase in the number of lymphocytes is known as lymphocytosis whereas a decrease is lymphocytopenia. A decrease in lymphocytes occurs when the human immunodeficiency virus HIV hijacks and destroys T cells specifically, the CD4+ subgroup of T lymphocytes. Without the key defense that these T cells provide, the body becomes susceptible to opportunistic infections that otherwise would not affect healthy people. The extent of HIV progression is typically determined by measuring the percentage of CD4+ T cells in the patient's blood. The effects of other viruses or lymphocyte disorders can also often be estimated by counting the numbers of lymphocytes present in the blood. See also Addressin Anergy Complete blood count Cytotoxicity Human leukocyte antigen Lymphoproliferative disorders Reactive lymphocyte Secretion assay References ^ a b Semester 4 medical lectures at Uppsala University 2008 by Leif Jansson ^ a b Janeway, Charles; Paul Travers, Mark Walport, and Mark Shlomchik 2001. Immunobiology; Fifth ion. New York and London: Garland Science. ISBN 0-8153-4101-6. . ^ a b c d Abbas AK and Lichtman AH 2003. Cellular and Molecular Immunology, 5th ed., Saunders, Philadelphia. ISBN 0-7216-0008-5. ^ Lymphocyte subsets in term and significantly preterm UK infants in the first year of life analysed by single platform flow cytometry 2005-05. Clin Exp Immunol 140 2: 289-292. doi:10.1111/j.1365-2249.2005.02767.x. inactive 2008-06-20. PMID 15807853. External links Histology at BU 01701ooa v d e Immune system: Lymphatic system Lymph, Lymphocytes Primary Bone marrow - Thymus Hassall's corpuscles Secondary: Spleen process blood Hilum - Trabeculae Red pulp Cords of Billroth, Marginal zone White pulp Periarteriolar lymphoid sheaths, Germinal center Trabecular arteries - Trabecular veins Secondary: Lymph nodes process extracellular fluid Afferent lymph vessels - Cortical sinuses - Medullary sinuses - Efferent lymph vessels T cells: High endothelial venules B cells: Primary follicle/Germinal center - Mantle zone - Marginal zone Lymph node capsule - Subcapsular sinus - Cortex - Paracortex - Medulla Medullary cord - Hilus Lymph node trabeculae Secondary: MALT process mucosa GALT - Peyer's patches - Germinal center v d e Blood General Plasma - Hematopoietic stem cells Lymphoid - WBC T cells: Cytotoxic CD8+, Helper CD4+/Regulatory, γδ, Natural Killer T cell B cells: Plasma, Memory Natural killer cells Lymphokine-activated killer cell Null cell Myeloid - WBC Monocytes/Macrophages Histiocytes, Kupffer cells, Langhans giant cells, Microglia, Osteoclasts, Epithelioid cells Granulocytes Neutrophil, Eosinophil, Basophil - Mast cell precursors Dendritic cells Langerhans cells, Follicular dendritic cells Megakaryoblast - Megakaryocyte - Platelets Myeloid - RBC Reticulocyte - Normoblast Retrieved from http://en..org/wiki/Lymphocyte Categories: LymphocytesHidden categories: Pages with DOIs broken since 2008 | Articles to be expanded since December 2007 | All articles to be expanded Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages Català Dansk Deutsch اردو Español Esperanto Euskara Français Bahasa Indonesia Italiano עברית МакедонÑ?ки Nederlands 日本語 Polski Português РуÑ?Ñ?кий Simple English SlovenÄ?ina СрпÑ?ки / Srpski Suomi Svenska Türkçe ä¸æ–‡ This page was last modified on 16 August 2008, at 10:13
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