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Subject - Immunology
About one third of all blood lymphocytes are T cells (thymus-dependent lymphocytes). When attracted by signalling substances, T cells can leave the bloodstream and enter tissues. They return to the lymph nodes by way of the lymph channels to start their rounds again. Each T cell recognizes only one epitope with its specific T cell receptor (one T cell, one antigen). The T cells are divided into the following subtypes:
Cytotoxic T cells (CD8+ cells) are directed toward infected cells and cancer cells. To that end, they constantly monitor the proteins produced by the body’s cells. Somatic cells present small peptide fragments of the proteins they generate bound to a receptor on the cell surface, the major histocompatibility complex class I (MHC I). If a cytotoxic T cell recognizes this sample as foreign, e.g. if the bound peptide is part of a viral shell, the CD8+ cell kills the cell being monitored and shifts the immune system into a high state of alert by flooding it with cytokines.
T helper cells (CD4+ cells) control and coordinate an immune response. They monitor the protein fragments presented by macrophages in the MHC II and decide if an immune response should be triggered. To do this, the activated and maturing T helper cell uses several receptors to come into direct contact with a B or killer cell, allowing both cells to communicate with each other with cytokines.
Memory T cells are deactivated but fully mature T cells that are held in reserve after an infection has been successfully withstood. They allow for a rapid response of the immune system upon renewed infection with the same pathogen. Memory T cells are an important component of the immunological memory formed through vaccination.
Regulatory T cells (Treg, CD25+ cells), formerly known as T suppressor cells, are supposed to inhibit immune reactions against the body’s own tissue, to maintain pregnancy, and to protect useful intestinal bacteria from attack by the immune system (Fehervari, Z.; Sakaguchi, S. (2006): Peacekeepers of the Immune System. In: Scientific American. , ). They are central players in the optimization of the immune reaction as well as avoidance of excessive reaction and thus minimization of the destruction of surrounding healthy tissue.
The term T cell derives from the word thymus, which is where the T precursor cells mature and where the T cells that are directed against the body’s own tissue are culled. T precursor cells are formed from lymphoid stem cells in the bone marrow.
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Our immune system is acting as a protective shield against infectious agents and microbial invaders. If this defence gets out of control and turns against the body’s own structures it may destroy healthy tissue and organs. The result is an autoimmune disease with severe or even life-threatening complications.