T Cells
Unlike B cells, T cells do not recognize free-floating antigens. Rather, their surfaces contain specialized antibody-like receptors that see fragments of antigens on the surfaces of infected or cancerous cells. T cells contribute to immune defenses in two major ways: Some direct and regulate immune responses, whereas others directly attack infected or cancerous cells.
Helper T cells, or Th cells, coordinate immune responses by communicating with other cells. Some stimulate nearby B cells to produce antibodies, others call in microbe-gobbling cells called phagocytes, and still others activate other T cells.
Cytotoxic T lymphocytes (CTLs)—also called killer T cells—perform a different function. These cells directly attack other cells carrying certain foreign or abnormal molecules on their surfaces. CTLs are especially useful for attacking viruses because viruses often hide from other parts of the immune system while they grow inside infected cells. CTLs recognize small fragments of these viruses peeking out from the cell membrane and launch an attack to kill the infected cell.
In most cases, T cells only recognize an antigen if it is carried on the surface of a cell by one of the body’s own major histocompatibility complex, or MHC, molecules. MHC molecules are proteins recognized by T cells when they distinguish between self and nonself. A self-MHC molecule provides a recognizable scaffolding to present a foreign antigen to the T cell. In humans, MHC antigens are called human leukocyte antigens, or HLA.
Although MHC molecules are required for T cell responses against foreign invaders, they also create problems during organ transplantations. Virtually every cell in the body is covered with MHC proteins, but each person has a different set of these proteins on his or her cells. If a T cell recognizes a nonself-MHC molecule on another cell, it will destroy the cell. Therefore, doctors must match organ recipients with donors who have the closest MHC makeup. Otherwise the recipient’s T cells will likely attack the transplanted organ, leading to graft rejection.
Natural killer (NK) cells are another kind of lethal white cell, or lymphocyte. Like CTLs, NK cells are armed with granules filled with potent chemicals. But CTLs look for antigen fragments bound to self-MHC molecules, whereas NK cells recognize cells lacking self-MHC molecules. Thus, NK cells have the potential to attack many types of foreign cells.
Both kinds of killer cells slay on contact. The deadly assassins bind to their targets, aim their weapons, and then deliver a lethal burst of chemicals.
T cells aid the normal processes of the immune system. If NK T cells fail to function properly, asthma, certain autoimmune diseases—including Type 1 diabetes—or the growth of cancers may result. NK T cells get their name because they are a kind of T lymphocyte that carries some of the surface proteins, called “markers,” typical of NK T cells. But these T cells differ from other kinds of T cells. They do not recognize pieces of antigen bound to self-MHC molecules. Instead, they recognize fatty substances (lipids and glycolipids) that are bound to a different class of molecules called CD1d. Scientists are trying to discover methods to control the timing and release of chemical factors by NK T cells, with the hope they can modify immune responses in ways that benefit patients.
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