Table of Contents
Definition of Cell-Mediated Immunity
The protective mechanism through which specialized immune cells, primarily T-cell lymphocytes and natural killer (NK) cells, carry out the immune response to protect the body from intracellular pathogens (diseasecausing 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 collaboratively with antibody-mediated immunity to protect the body from infection.
T-cell lymphocytes and Cell-Mediated Immunity
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 longterm 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 and Cell-Mediated Immunity
Macrophages set cell-mediated immunity in action when they display the antigens of a consumed 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 cytotoxic 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 transplantation. Cell-mediated immunity, with its focus on nonself antigens, is key to graft vs. host disease and organ transplant rejection.
NK Cells and Cell-Mediated Immunity
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 cytokines that weaken the cell membrane 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. However, researchers do not understand the mechanisms of this recognition or to what extent NK cells are able to suppress the growth of cancer cells.
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