Lymphocytes Thymus

T-cells. T-lymphocytes thymus-dependent lymphocytes the cells primarily responsible for cell-mediated immunity. 

T lymphocytes, thymus-derived lymphocytes or T cells are formed in the thymus. They carry out immune reactions involving cell-cell interactions and are responsible for cell-mediated immunity. One distinguishes specific and non-specific cytotoxic killer T cells, (NK cells, natural killer cells), and helper T cells which cooperate with antigen-presenting cells, APC s, in the initiation of an immune response. Suppressor T cells dampen the action of helper T cells. 

T-lymphocyte (thymus dependent T-cell) A type of lymphocyte involved in cellular immune reactions and aiding in the production of antibodies. They originate in haemopoietic stem cells, but undergo essential maturation in the thymus gland. They interact with other cells (e.g. B-lymphocytes) and e.g. antigens, lymphokines via receptor sites on their membranes. There are several subsets of T-lymphocytes see cytotoxic T-cells helper T-cells inducer T-cells suppressor T-cells. 

T-Lymphocyte Thymus-derived cell of the immune system and agent of cellular immune responses. 

T-lymphocytes [thymus derived and posess characteristic surface receptors] and 5) macrophages [mononuclear phagocytic cells]. 

In addition to antibodies, the immune system also consists of bone-marrow derived lymphocytes, or B cells, and T cells that come from the thymus gland, both of which (indirectly) produce antibodies. These cells, in turn, may be helped by helper cells (= H) and suppressed by suppressor cells (= S). Helper cells may be alarmed as to the presence of antigens by macrophages (= M) that eat the antigens and leave parts of their meal on their cell surface. 

This class of lymphocytes differentiates from immuno-logically incompetent hematopoietic stem cells of the bone marrow within the thymus - hence, the name thymus-dependent (T-) lymphocytes. Two major subclasses develop simultaneously, T-helper lymphocytes (Th) and cytotoxic effector lymphocytes (Tc). The cytotoxic T-lymphocytes (carrying on the surface the differentiation marker CD8) destroy cells, which cany their cognate antigen bound to MHC class I molecules on the surface by inducing apoptosis. From an evolutionary point of view Tc cells appear to have developed predominantly to cope with vims infections. As vituses can only replicate within cells, Tc eliminate them by destroying their producers. 

The thymus is a lympho-epithelial organ, located within the upper thorax. One of its fimctions is the conversion of certain lymphoid hematopoietic precursor cells originating from the bone marrow into thymus-derived lymphocytes (T-cells). 

Decreases the production of lymphocytes and eosinophils in the blood by causing atrophy of the thymus gland blocks the release of cytokines, resulting in a decreased performance of T and B monocytes in the immune response. (This action, coupled with the anti-inflammatory action, makes the corticosteroids useful in delaying organ rejection in patients with transplants.)... 

Miller Scott (1985) reported marked reduction in thymus weight in rats fed dioctyltin dichloride for 8 or 12 weeks at a level of 75 mg/kg diet. Numbers of lymphocytes together with T cell subpopulations were reduced in treated rats, but no difference was seen in antibody response to sheep red blood cells in vivo. No evidence was foimd of in vitro cytocidal effects of dioctyltin dichloride on blood lymphocytes. Evans et al. (1986) dosed pregnant and non-pregnant rats for 3 weeks at 75 mg/kg diet and reported severe thymic atrophy and extensive vacuolation of reticuloendothelial cells in pregnant animals only. 

Adenosine deaminase deficiency is associated with an immunodeficiency disease in which both thymus-derived lymphocytes (T cells) and bone marrow-derived lymphocytes (B cells) are sparse and dysfunctional. Purine nucleoside phosphorylase deficiency is associated with a severe deficiency of T cells but apparently normal B cell function. Immune dysfunctions appear to result from accumulation of dGTP and dATP, which inhibit ribonucleotide reductase and thereby deplete cells of DNA precursors. 

Key Words Chemokines lymphocyte development T-cell differentiation cell trafficking memory thymus bone marrow. 

Ueno T, Hara K, Willis MS, et al. Role for CCR7 ligands in the emigration of newly generated T lymphocytes from the neonatal thymus. Immunity 2002 16 205-218. 

Matloubian M, Lo CG, Cinamon G, et al. Lymphocyte egress from thymus and peripheral lymphoid organs is dependent on SIP receptor 1. Nature 2004 427 355-360. 

Kunkel EJ, Campbell JJ, Haraldsen G, et al. Lymphocyte CC chemokine receptor 9 and epithelial thymus-expressed chemokine (TECK) expression distinguish the small intestinal immune compartment Epithelial expression of tissue-specific chemokines as an organizing principle in regional immunity. J Exp Med... 

Purine nucleoside phosphorylase (PNP) deficiency engenders a combined immunodeficiency and neurologic abnormalities and is usually fatal in childhood (G4). Patients with PNP deficiency have profound lymphopenia and a small thymus with poorly formed Hassall corpuscles. Lymphocyte enumeration shows markedly decreased numbers of T cells and T-cell subsets, with normal percentages of B cells. Point mutations and a splicing mutation have been identified in some PNP-deficient patients (H4). 

In the periphery, CB1 is found in the adrenal glands, bone marrow, heart, lung, prostate, testes, thymus, tonsils, spleen, lymphocytes, phagocytes, smooth muscle, vascular endothelium, peripheral neurons (e.g., in the gut), kidneys, uterus, and sperm as reviewed by Schuel et al. (1999). 

The CB2 receptor has a more limited distribution, being localized predominantly in the immune system. Among the human leukocytes, B lymphocytes express the highest levels of CB2, followed respectively by natural killer cells, monocytes, polymorphonuclear neutrophils, T8 lymphocytes, and T4 lymphocytes. It is also found in the lymph nodes, spleen, tonsils, and thymus (Cabral, 1999). 

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