What is a T cell?

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. 

What is an antigen?

The term antigen refers to something that is recognized by the immune system. This can be something on the surface of a virus, a pollen grain, a toxin produced by a bacteria, or any number of other small molecules. Antigens can come from the environment and be foreign to the body, or they can be something that is normally produced by cells of the body. These latter antigens are called self antigens or autoantigens, and the response of the immune system against them is referred to as autoimmunity.