T cell receptors are complex protein molecules that peek through the surface membranes of T cells. The exterior part of a T cell receptor recognizes short pieces of foreign antigens that are bound to self-MHC molecules on other cells of the body. It is because of their T cell receptors that T cells can recognize disease-causing microorganisms and rally other immune cells to attack the invaders, or kill the invaders themselves.
Toll-like receptors (TLRs), which occur on cells throughout the immune system, are a family of proteins the body uses as a first line of defense against invading microbes. Like T cell receptors, some TLRs peek through the surface membranes of immune cells, allowing them to respond to microbes in the cells’ environment.
Some TLRs are activated by molecules that make up viruses, whereas other TLRs respond to molecules that make up the cell walls of bacteria. Once activated, TLRs relay the alarm to other actors in the immune system. For example, some TLRs play important roles in the all-purpose “first-responder” arm of the immune system, also called the innate immune system. In short order, the innate immune system responds with a surge of chemical signals that together cause inflammation, fever, and other responses to infection or injury. Other TLRs help initiate responses from genetically identical groups of lymphocytes, called clones, that are already programmed to recognize specific antigens. Such responses are called adaptive immunity.
Overall, the cellular receptors important for the first-line responses of innate immunity are encoded by genes people inherit from their parents. In contrast, adaptive immune responses rely on antigen receptors that are pieced together in the genomes of lymphocytes during their development in various tissues of the body. In addition to TLRs, other kinds of innate immune receptors can stimulate phagocytosis by macrophages, trigger the inflammatory responses that help control local infections, and play a range of crucial roles in defending the body against invading microbes.
Last Updated April 29, 2008