T lymphocytes, also known as T cells, are white blood cells that form part of the immune system. Together with B lymphocytes, T lymphocytes help protect the body from infection by recognizing and destroying threatening particles such as bacteria. On the surfaces of lymphocytes are special receptors which bind to a specific foreign particle, or antigen. There are two main types of T lymphocytes, known as helper T cells and killer T cells. Helper T cells activate killer T cells, which then destroy threats such as microbes and cancer cells.
A T lymphocyte is a particular type of white blood cell known as an agranular cell, which is filled with a clear cytoplasm, or gel; B cells are also agranular. Granular lymphocytes exist, but they make up only a small proportion of all lymphocytes. They include natural killer cells, which have granules in their cytoplasm, containing chemicals which may be released to destroy target cells. Lymphocytes are produced in the bone marrow and then travel to different parts of the lymphatic system. T lymphocytes travel to, and mature inside, the thymus gland, which is why they are referred to using the letter T for thymus.
Helper T lymphocytes help to drive the immune system by activating killer T cells and B cells. Before this can take place, the helper T lymphocytes themselves require activation. Usually, T cells are not able to respond to antigens which are free in the body, but they do recognize antigens on the surfaces of cells which have been infected or become cancerous.
Macrophages are cells which are able to consume antigens. The antigens then move to the outside of the cell, and a macrophage may display parts of them on its surface. When these antigen fragments match the receptors on a helper T lymphocyte, the lymphocyte recognizes them, binds to them and becomes activated.
Activated helper T cells multiply and secrete proteins, which stimulate other immune system cells including B cells and killer T cells. Killer T lymphocytes are able to recognize bacteria and virus-infected cells and cancer cells. They are sometimes referred to as cytotoxic T lymphocytes. Receptors on the surfaces of killer T cells recognize and bind to fragments of antigen on the surfaces of cancer cells and infected cells. It is thought that killer T cells could be involved in the development of autoimmune diseases, such as type 1 diabetes, where they could mistakenly destroy insulin-producing cells.