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 sur-
face 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 anti-
bodies 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 differ-
entiation (CD). There are two types of clusters of differentiation. These are:- CD4—Helper T cells
- CD8—Cytotoxic T cells and suppressor T cells
Macrophages and Natural Killer Cells
Macrophagesare phagocytic cells that ingest antigens. They are in a resting state
until they receive a cytokine from the helper T cell, at which point they become
large and ruffled and ready to attack the antigens. Macrophages destroy virus-
infected cells and bacteria in intracellular locations. They also eliminate some
cancerous cells.
Natural killer cells (NK)are lymphocytes that destroy other cells such as
tumor cells. Natural killer cells are always active and searching for an infected
cell. These are different from other cells in the immune system, which become
activated only when stimulated by an antigen.(^220) CHAPTER 14 Immunity