Certain antigens, such as those derived from the agents responsible for tuberculosis, mycoses, and some viral diseases elicit primarily a cell mediated immune response. Antigens in contact with "T" lymphocytes under proper conditions, stimulate their differentiation into blast cells, and their secretion of factors (lymphokines) that act non-specifically on a variety of other cells. These lymphokines aid in the elimination of infected agents by attracting phagocytic cells to sites of infection (Macrophage Chemotactic Factor, MCF) and holding them there (Migration Inhibition Factor, MIF).
Certain "T" lymphocytes that are primed to respond to viral, tumor or histocompatibility antigens can cause lysis of cells on the surfaces of which the corresponding antigens are expressed. This specific cytolysis is independent of antibody and complement and the cells are called cytotoxic T lymphocytes (CTL).
Other lymphocytes, designated "K" (killer) cells can cause lysis of target cells only if the latter are coated with antibody. "K" lymphocytes are specific with respect to their target, but nonspecific with respect to their mechanism of killing. Other non- specific lymphoid cells are normally present in the host without prior exposure to antigen. These "natural killer" cells (NK) lyse tumor or virus infected cells without antibody.
The most common method for monitoring development and maintenance of cellular immunity is skin testing, which is used for such antigens as tuberculin, candida or histoplasmin. More specific in vitro tests include those that involve phytohemagglutinin, purified antigens, or other lymphocytes, thus demonstrating cellular reactivity.
SUMMARY
T cell specific for antigen (Ag) enters
circulation
/
T cell encounters and combines with Ag
/
T cell becomes activated by Ag
/
Activated T cell returns to
lymphoid tissue
/
Proliferation (clonal expansion) of T cells
for
Ag occurs
/
T cells for Ag enter circulation in relatively large
numbers
/
Some T cells (from expanding clones) contact Ag
and react
with it
/
Lymphokines (soluble mediators) are released
/
Lymphokines contribute to production of cell-mediated immunity
or
delayed-type hypersensitivity
