C
cell-mediated immunity The protective mecha-
nism through which specialized immune cells, pri-
marily T-cell lymphocytes and natural killer (NK)
cells, carry out the IMMUNE RESPONSE to protect
the body from intracellular pathogens (disease-
causing entities, such as viruses and parasites, that
invade the body’s cells). Cell-mediated immunity
encompasses cytotoxic (death of invading cells)
and phagocytic (consumption of cellular debris)
activities. Cell-mediated immunity functions col-
laboratively with ANTIBODY-MEDIATED IMMUNITY to
protect the body from INFECTION.
Several kinds of T-cell lymphocytes participate
in cell-mediated immunity. They include
- cytotoxic T-cells, which respond directly to
antigens for which they are sensitized and kill
the cells that bear them - helper T-cells (Th1 cells), also called CD-4 cells,
which release CHEMOKINES in response to the
presence of the antigen-bearing cells - memory T-cells, which are essential for long-
term immunity against infections such as
MEASLES and POLIOMYELITIS (activated through
disease or vaccination) - suppressor T-cells, which bring the immune
response to a close when the threat is gone
Macrophages set cell-mediated immunity in
action when they display the antigens of a con-
sumed cell. When these are nonself antigens,
cytotoxic T-cells respond to kill other cells bearing
the ANTIGEN. When the antigen is one the body has
previously encountered, memory T-cells sensitized
to the particular antigen rapidly convert to cyto-
toxic T-cells and mount a fast-strike immune
response. The ability of cell-mediated immunity to
rid the body of nonself-antigen-bearing cells is
highly effective for controlling infection though
also becomes problematic in ORGAN TRANSPLANTA-
TION. Cell-mediated immunity, with its focus on
nonself antigens, is key toGRAFT VS. HOST DISEASE
and organ transplant rejection.
The NK cell, a type of granular LEUKOCYTE, does
not respond to antigens. Rather, it responds to
MAJOR HISTOCOMPATIBILITY COMPLEX(MHC) molecules
on the surfaces of cell membranes. Attacking NK
cells produce CYTOKINESthat weaken the cell mem-
brane of the targeted cells, which indirectly causes
the death of the cells. NK cells also respond to
tumor antigens and are particularly active in
killing cancer cells. Researchers believe NK cells
have a limited ability to recognize changes in cells
that alter cellular identity (altered self), such as
those occurring when cells turn cancerous. How-
ever, researchers do not understand the mecha-
nisms of this recognition or to what extent NK
cells are able to suppress the growth of cancer
cells.
See also HYPERSENSITIVITY REACTION; MACROPHAGE;
MONONUCLEAR PHAGOCYTE SYSTEM; NATURAL KILLER
(NK) CELL; PARASITE; PATHOGEN; PHAGOCYTE; PHAGOCY-
TOSIS; T-CELL LYMPHOCYTE; VACCINE; VIRUS.chemokines Proteins, also called chemotactic
CYTOKINES, that draw or direct leukocytes to the
scene of INFECTIONwithin the body. Macrophages
produce chemokines when they encounter foreign
cells, instigating an IMMUNE RESPONSE. Some
chemokines act as homing signals, marking the
foreign cells so responding immune cells can zero
in on them. Other chemokines send out biochemi-
cal “alerts” that attract circulating monocytes and
lymphocytes.
Chemokines are integral in the process of
angiogenesis (the development and growth of253