T cells are a subset of lymphocytes that play a large role in the primary immune response. The abbreviation "T" stands for thymus, the organ in which their final stage of their development occurs.

There are many types of T cells:

  • Cytotoxic T cells (CD8+) destroy infected cells. These cells function as 'killer' or cytotoxic cells because they are able to destroy target cells which express specific antigens that they recognize.
  • Helper T cells (CD4+) are "middlemen" in the immune response and proliferate to activate many other types of cells which act more directly in the response. CD4+ T cells regulate or 'help' other lymphocyte function. They are known as the target of HIV infection, and the decrease of CD4+ T cells results in AIDS.
  • Suppressor T cells turn off the immune response once an antigen has been eliminated from the body.

Cell-mediated immunity (the immune response against pathogenic organisms inside body cells) involves T cell activation.

See also: