Cell-mediated immunology is the immune system's response to cellular infection or malfunction. It operates through various types of lymphocytes,or white blood cells, that circulate throughout the body. Chief among these are certain types of T-cells that bind to infected cells and cause them to die by a series of chemical changes. Also called cell-mediated immunity, it raises defenses against viral and bacterial infection, and can initiate destruction of cancerous cells. Removing intracellular parasites is another function.
There are two major systems of immune response in the body. Antibody-mediated immunity relies on specialized B-cells that float in fluids like blood and lymph, attaching to a foreign microbe, called an antigen, and initiating a response to contain and destroy it. Conversely, cell-mediated immunity is the response of T-cells and various lymphocytes that respond to compromised cells, which they recognize and subsequently destroy. Cell-mediated immunology can also refer to the study of these molecular and cellular immune elements.
Antigen-specific T-cells recognize a pathogen already labeled by the immune system during infection. If these pathogens enter the body's own cells, small pieces of the foreign organism, called epitopes, are displayed on the cell. In the case of cancerous cells, tumor antigens may be present despite the absence of foreign matter, since cancer changes cellular identity. Once T-cells find an epitope, they induce a lethal chemical cascade within the infected or cancerous cell. Cell-mediated immunology also incorporates dozens of chemical agents called cytokines that regulate the growth and movement of lymphocytes.
One important role of cell-mediated immunology is the destruction of parasites living within cells. Some bacteria and protozoans move through the membrane of cells and hide from the B lymphocytes there. Sometimes these pathogens are engulfed by lymphocytes called macrophages, but manage to stay alive even within this hostile intracellular environment. The macrophages, however, present pieces of these organisms as epitopes on specialized proteins embedded in the cell surface. This alerts the CD4 T-cells, which then release chemicals enabling the macrophage to kill whatever pathogen it has ingested.
Antiviral activity is another basic component of cell-mediated immunology. Almost all of the body's cells can be infected with some virus. Cells have specialized proteins that display fragments of viruses as epitopes, attracting T cells. CD4 T cells do not directly destroy infected or cancerous cells, but rather facilitate this work by other immune system effectors. The different functions of cell-mediated immunity are carried out by specialized types of lymphocyte, in addition to T cells, including the macrophages, and the natural killer cells. Macrophages engulf and digest pathogens while natural killer cells destroy the body's own infected or cancerous cells, which they recognize as having changed from their natural state.