T-cells are a type of white blood cell (leukocyte) that belongs to a division of the immune system called the acquired immune system. This is the part of the immune system that learns to combat invading bacteria and viruses (pathogens) through exposure to them. The leukocytes that belong to the acquired immune system are called lymphocytes and have receptors on their surface that recognise broken down protein fragments, called antigens, by binding with them. Antigens can be derived either from invading pathogens or, in the case of autoimmune diseases like multiple sclerosis, from cells of one's own body.
The acquired immune system is further divided into cell-mediated or cellular immunity and humoral or antibody-mediated immunity. T-cells are the major cells that drive cellular immunity whereas an another type of lymphocyte, called a B-cell, is the principle cell involved with antibody-mediated immunity. T-cells are so-called because they are matured in an organ called the Thymus.
The surface of a T-cell contains thousands of T-Cell Receptors (TCR) but, for any one T-cell, all the receptors are identical (monoclonal). This means that any one T-cell is only able to recognise a small group of related antigens i.e. each T-cell is specific only to those antigens and is not effective against any others. The receptor rarely binds with an entire antigen but with a sub-section of it called an epitope.
The job of T-cells is to detect cells in the body that are internally infected with viruses and bacteria. They do this by sampling the contents of cells. Two types of T-cells sample different populations of cells and take different action when they detect a antigen. These are the "killer" or cytotoxic (CD8+) T-cells and the "helper" (CD4+) T-cells. The CD8+ and CD4+ describe the types of receptors that each carries. A third type of T-cell called a "suppressor" T-cell also uses the CD8+ receptor.
Almost all the cells in the body express a protein called the Major Histocompatibility Complex (MHC) protein. The function of MHC is to present antigens to T-cells. MHC has a slit in it shaped like a letter box and the cell pushes antigens through this slit. T-cell receptors plug onto the MHC molecule and try to bind with the presented antigen. MHC comes in two major varieties: MHC class I and MHC class II.
MHC class I is present on almost all cells and it is the job of killer T-cells to bind to antigens presented in this way. When a match is found, the killer T-cell latches onto the infected cell and destroys it. How this is done is dealt with in the entry for killer T-cells.
MHC class II is present only on a population of cells known as antigen presenting cells (APC). These include macrophages, B-cells and dendritic cells. It is the job of helper T-cells to bind to antigens presented in this way. When this happens, a helper T-cell can do several things:
The third type of T-cell, the suppressor T-cell, is involved with suppressing an immune response. It is not well understood how they do but they probably use several mechanisms including "programmed cell death" (apoptosis) which involves sending cytokines to other immune system cells telling them to commit suicide. Suppressor T-cells are dealt with more fully in the entry for suppressor T-cells.
T-cells are manufactured in the bone marrow but migrate to an organ called the thymus where they are matured via a process called affinity maturation which removes those which are active against the body's own antigens (autoreactive). Selection for particular T-cells is dealt with in the entry on the thymus.
HON Allergy Glossary T-Cell
B Cells and T Cells
Immunology and Immune Defense against Microbial Pathogens