A T cell is a type of lymphocyte that uses a receptor on its surface to recognize fragments of proteins (antigens) that are bound to a molecule called major histocompatibility complex (MHC). The 'T' in their name refers to the fact that they go through a selection process in an organ called the thymus early in their development. Selection occurs in two major steps in which the quality of the T cell receptor is tested by positive binding to MHC (positive selection), and subsequently tested for reactivity against self antigens (negative selection). T cells that bind self antigens too strongly create a risk of driving autoimmune diseases, so the result of negative selection is most often to kill the self-reactive T cell. However, some T cells escape the death process and instead are either programmed to be unresponsive (anergic) or they are educated to become suppressors or down regulators of the immune system. These 'regulatory' T cells (Tregs) are a very important source of protection from autoimmune diseases and other types of hyper immune reactions.
There are two major subsets of T cells that are distinguished by the types of antigen/MHC they recognize and their functions. Cytotoxic T lymphocytes (CTL) recognize antigens that are presented by Class I MHC, and T helper lymphocytes (TH cells) recognize antigens that are bound to Class II MHC. A T cell entering the thymus has the capacity to become either of these types of T cell, and the decision is based on interactions that occur during positive selection. When a T cell has a preference for binding to Class II MHC, then a second molecule known as CD4 also attaches to the MHC acting as an anchor to enhance the interaction. Conversely, binding to Class I MHC is anchored by a molecule called CD8, and when either CD4 or CD8 become activated on the T cell in the thymus, the expression of the other molecule is shut off. Thus, CTL are also known as CD8+ T cells and helper T cells are called CD4+ T cells, and these two markers can be very useful in identifying the type of T cells that are present in a sample.
The two types of T cells have very different functions in the immune system. CD8+ CTL are specialized in recognizing antigens that come from the inside of a target cell. This is important for detecting viruses and bacteria that infect and live inside of cells. It is also a mechanism used to identify cancer cells that often have alterations in their intracellular contents. When CTL become activated by binding antigen on Class I MHC, their main function is to kill the cell with which they are interacting. CD4+ TH cells have a much more complex role that is worthy of a separate description, but suffice it to say for now that they are the major orchestrators of all immune responses.