When a vertebrate encounters substances that are capable of causing it harm, a protective system known as the "immune system" comes into play. This system is a network of many different organs that work together to recognize foreign substances and destroy them. The immune system can respond to the presence of a disease causing agent (pathogen) in two ways. Immune cells called the B cells can produce soluble proteins (antibodies) that can accurately target and kill the pathogen. This branch of immunity is called "humoral immunity." In cell-mediated immunity, immune cells known as the T cells produce special chemicals that can specifically isolate the pathogen and destroy it.
The T cells and the B cells together are called the lymphocytes. The precursors of both types of cells are produced in the bone marrow. While the B cells mature in the bone marrow, the precursor to the T cells leaves the bone marrow and matures in the thymus. Hence the name, "T cells" for thymus-derived cells.
The role of the T cells in the immune response is to specifically recognize the pathogens that enter the body and to destroy them. They do this either by directly killing the cells that have been invaded by the pathogen, or by releasing soluble chemicals called "cytokines," which can stimulate other killer cells specifically capable of destroying the pathogen.
During the process of maturation in the thymus, the T cells are taught to discriminate between "self" (an individual's own body cells) and "non-self" (foreign cells or pathogens). The immature T cells, while developing and differentiating in the thymus, are exposed to the different thymic cells. Only those T cells that are "self-tolerant," that is to say, they will not interact with the molecules normally expressed on the different body cells are allowed to leave the thymus. Cells that react with the body's own proteins are eliminated by a process known as "clonal deletion." The process of clonal deletion ensures that the mature T cells, which circulate in the blood, will not interact with or destroy an individual's own tissues and organs. The mature T cells can be divided into two subsets, the T-4 cells (that have the accessory molecule CD4) or the T-8 (that have CD8 as the accessory molecule).
There are millions of T cells in the body. Each T cell has a unique protein structure on its surface known as the "T cell receptor" (TCR), which is made before the cells ever encounter an antigen. The TCR can recognize and bind only to a molecule that has a complementary structure. It is kind of like a "lock-and-key" arrangement. Each TCR has a unique binding site that can attach to a specific portion of the antigen called the epitope. As stated before, the binding depends on the complementarity of the surface of the receptor and the surface of the epitope. If the binding surfaces are complementary, and the T cells can effectively bind to the antigen, then it can set into motion the immunological cascade which eventually results in the destruction of the pathogen.
The first step in the destruction of the pathogen is the activation of the T cells. Once the T lymphocytes are activated, they are stimulated to multiply. Special cytokines called interleukins that are produced by the T-4 lymphocytes mediate this proliferation. It results in the production of thousands of identical cells, all of which are sp ecific for the original antigen. This process of clonal proliferation ensures that enough cells are produced to mount a successful immune response. The large clone of identical lymphocytes then differentiates into different cells that can destroy the original antigen.
The T-8 lymphocytes differentiate into cytotoxic T-lymphocytes (CTLs) that can destroy the body cells that have the original antigenic epitope on its surface, e.g., bacterial infected cells, viral infected cells, and tumor cells. Some of the T lymphocytes become memory cells. These cells are capable of rememberin g the original antigen. If the individual is exposed to the same bacteria or virus again, these memory cells will initiate a rapid and strong immune response against it. This is the reason why the body develops a permanent immunity after an infectious disease.
Certain other cells known as the T-8 suppressor cells play a role in turning off the immune response once the antigen has been removed. This is one of the ways by which the immune response is regulated.