T Cells

T Cell Lymphocytes

Killer T Cell
A killer T cell lymphocyte (bottom) is attacking a cancer cell (top). Credit: Coneyl Jay/Stone/Getty Images

T Cells

T cells are a type of white blood cell known as a lymphocyte. Lymphocytes protect the body against cancerous cells and cells that have become infected by pathogens, such as bacteria and viruses. T cell lymphocytes develop from stem cells in bone marrow. These immature T cells migrate to the thymus via the blood. The thymus is a lymphatic system gland that functions mainly to promote the development of mature T cells.

In fact, the "T " in T cell lymphocyte stands for thymus derived. T cell lymphocytes are necessary for cell mediated immunity, which is an immune response that involves the activation of immune cells to fight infection. T cells function to actively destroy infected cells, as well as to signal other immune cells to participate in the immune response.

T Cell Types

T cells are one of three main types of lymphocytes. The other types include B cells and natural killer cells. T cell lymphocytes are different from B cells and natural killer cells in that they have a protein called a T-cell receptor that populates their cell membrane. T-cell receptors are capable of recognizing various types of specific antigens (substances that provoke an immune response). Unlike B cells, T cells do not utilize antibodies to fight germs.

There are several types of T cell lymphocytes, each with specific functions in the immune system.

Common T cell types include:

  • Cytotoxic T cells (also called CD8+ T cells) - are involved in the direct destruction of cells that have become cancerous or are infected with viruses. Cytotoxic T cells contain granules (sacs containing digestive enzymes or other chemical substances) that they utilize to cause the target cell to burst open in a process called apoptosis. These T cells are also the cause of transplant organ rejection. The T cells attack the foreign organ tissue as the transplant organ is identified as infected tissue.
  • Helper T cells (also called CD4+ T cells) - precipitate the production of antibodies by B cells and also produce substances that activate cytotoxic T cells and white blood cells known as macrophages. CD4+ cells are targeted by HIV. HIV infects helper T cells and destroys them by triggering signals that result in T cell death.
  • Regulatory T cells (also called suppressor T cells) - suppress the response of B cells and other T cells to antigens. This suppression is needed so that an immune response does not continue once it is no longer needed. Defects in regulatory T cells can lead to the development of an autoimmune disease. In this type of disease, immune cells attack the body's own tissue.
  • Natural Killer T (NKT) cells - have a similar name as a different type of lymphocyte called a natural killer cell. NKT cells are T cells and not natural killer cells. NKT cells have properties of both T cells and natural killer cells. Like all T cells, NKT cells have T-cell receptors. However, NKT cells also share several surface cell markers in common with natural killer cells. As such, NKT cells distinguish infected or cancerous cells from normal body cells and attack cells that do not contain molecular markers that identify them as body cells. One type of NKT cell known as an invariant natural killer T (iNKT) cell, protects the body against obesity by regulating inflammation in adipose tissue.
  • Memory T cells - help the immune system to recognize previously encountered antigens and respond to them more quickly and for a longer period of time. Helper T cells and cytotoxic T cells can become memory T cells. Memory T cells are stored in the lymph nodes and spleen and may provide lifetime protection against a specific antigen in some cases.

T Cell Activation

T cells are activated by signals from antigens they encounter. Antigen-presenting white blood cells, such as macrophages, engulf and digest antigens. Antigen-presenting cells capture molecular information about the antigen and attach it to a major histocompatibility complex (MHC) class II molecule. The MHC molecule is then transported to the cell membrane and presented on the surface of the antigen-presenting cell. Any T cell that recognizes the specific antigen will bind to the antigen-presenting cell via its T-cell receptor.

Once the T-cell receptor binds to the MHC molecule, the antigen-presenting cell secretes cell signaling proteins called cytokines. Cytokines signal the T cell to destroy the specific antigen, thus activating the T cell. The activated T cell multiplies and differentiates into helper T cells. Helper T cells initiate the production of cytotoxic T cells, B cells, macrophages, and other immune cells to terminate the antigen.