T cells develop from stem cells in bone marrow and migrate to the thymus gland where they mature. They then migrate to the lymphatic system to begin their fight against antigens. The "T" stands for thymus gland. Once the organism reaches late adulthood, the ability to create new T cells diminishes, resulting in a weaker immune system as the organism ages.
A T cell attacks a specific antigen that is displayed on the surface of the cell. These cells are called antigen-presenting cells (APCs), such as, macrophages and dendritic cells. After the antigen is ingested by the APC, fragments of the antigen are placed on the surface of the cell. These fragments must be near the cell-surface self-molecules. Self-molecules are part of the histocompatibility complex (MHC), which is a group of proteins that are unique to a person and used to distinguish self from nonself.
When an antigen receptor encounters fragments of the complimentary antigen, the T cell transforms into the effector T cell that carries out the immune response. An effector T cell is an antigen-stimulated cell. Some T cells attack the antigen in a primary immune response, while others become memory cells and take on a secondary immune response role when the antigen is encountered later on.
There are four types of T cells, each identified by characteristics of their surface molecules. These are
• Helper T(TH) cells. These cause the formation of cytotoxic T cells, activate macrophages, produce cytokines, and are essential to the formation of antibodies by B cells.
• Cytotoxic T (TC) cells. These destroy cells that have been infected by viruses and bacteria.
• Delayed hypersensitivity T (TD) cells. These are associated with allergic reactions.
• Suppressor T (TS) cells. These turn off the immune response when there are no antigens.
T cells are also identified by their surface receptors, called clusters of differentiation (CD). There are two types of clusters of differentiation. These are:
• CD8—Cytotoxic T cells and suppressor T cells
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