T cell

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.

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