T Cells in the Immune SystemLymphocyte:
Cells of the immune system responsible for defense reactions of the body against foreign substances he considers.
Cells belong to the family of leukocytes (white cells), representing approximately 20 to 30% of blood leukocytes, or 1 000 to 4 000/mm 3. They are distinguished by their small size (7 to 9 microns in diameter), their nucleus, round or ovoid, and their cytoplasm, scarce and poor in granules.
There are several types of cell, defined by both their function and membrane markers, designated according to the nomenclature CD (cluster of differentiation, or class differentiation) followed by a number. These markers are molecules defines a group of cells with common properties. These different types derived from the same precursors, lymphoid stem cells from bone marrow hematopoietic.
These immune system cells represent about 10% of lymphocytes circulating in the blood and develop in the bone marrow (bone marrow in English, hence the name). B cells are responsible for humoral immune response: they are specialized in the production of antibodies, they secrete after being transformed into plasma cells and that play in the "humors" (liquid) from the body. Their activation is a multistep process: B lymphocytes carry immunoglobulins - or antigen receptors - on their cytoplasmic membrane, each cell with a type of immunoglobulin of its own. When one of them encounters a circulating antigen, complementary to the immunoglobulin is the signal for him that he must produce antibodies (immunoglobulins identical to those of its membrane, but in soluble form) to fight against this foreign antigen. Lymphocytes from all divisions of such a B cell clone formed a group called and are endowed with the same specificity and the same mission as the parent cell.
These immune system cells whose maturation occurs in the bone marrow and the thymus - hence the name - gland at the top of the chest behind the breastbone. T cells differentiate into two populations responsible for the cellular immune response.
- The auxiliary CD4 T lymphocytes, or T "helper" or T4, are specialized in the secretion of cytokines (including interleukins), molecules enabling them to cooperate with other cells, which are responsible for the elimination of antigens.
- The CD8 T cells or T8, include two types of cells: cytotoxic lymphocytes capable of killing cancer cells, foreign or those infected with a virus, and suppressor T cells, whose role is to control immune responses.
As B cells, T cells have membrane molecules - antigen receptors - adapted to the recognition of the antigen against which they struggle. But it must be presented to them by a specialized cell such as a macrophage, which degrades the antigen and prepares the fragments by combining them with molecules that have a say, the molecules of the major histocompatibility complex (MHC). Them, vary from one individual to another, also determine the potential for graft between two subjects.
- NK cells (natural killer of English, natural killer) cells are related to T cells, with whom they share some membrane markers. These cells are endowed with natural cytotoxic activity, they exert spontaneously to destroy cells infected by viruses or cancer cells. Their properties can be enhanced by interleukins secreted by T cells, is referred to as LAK cells (lymphokine activated killer, killer activated by lymphokines). The use of these cells in the treatment of certain cancers is still experienced.
Lymphocytes are explored quantitatively by blood count, the study of their distribution between different types based on the expression of membrane molecules of differentiation, as evidenced by highly sophisticated techniques now (immunophenotyping). There is also the most functional exploration methods, which promote the ability to study cell response to activation signals.
Cells can proliferate (leukemia), decrease in number or have functional abnormalities (congenital or acquired immune deficiency such as AIDS). There are also many diseases related to various cell dysfunctions such as autoimmune diseases (multiple sclerosis, diabetes mellitus, systemic lupus erythematosus, etc..) Or allergic (asthma, hay fever).