T Cells

T lymphocytes (T cells) are made in the bone marrow but mature in the thymus, a gland in the chest. Each T cell has T cell receptors on its surface, all with the same shape, complementary to one specific antigen.Unlike B cells, T cells cannot recognise free antigens in body fluids. A T cell receptor only binds to an antigen fragment when it is presented on an MHC protein on the surface of another cell. This may be an antigen-presenting cell such as a macrophage, or a body cell infected by a virus or other pathogen. This means T cells can respond to pathogens inside body cells, where antibodies cannot reach them.When a T cell binds to a complementary presented antigen, it is activated and divides by mitosis to form a clone. The clone differentiates into several types:
• T helper cells release cytokines (interleukins) that stimulate B cells to divide and make antibodies, activate T killer cells and increase phagocytosis by macrophages. They coordinate the whole immune response.
• T killer cells (cytotoxic T cells) bind to infected body cells and destroy them.
• T memory cells remain in the body for many years and divide rapidly if the same antigen appears again.
• T regulatory (suppressor) cells suppress the immune response once the infection is controlled, preventing damage to the body's own tissues.HIV infects and destroys T helper cells. Without them, B cells and T killer cells are not activated, which is why people with AIDS suffer many other infections.