From these humble beginnings, the field of mammalian toll-like receptors (TLR) quickly evolved as a crucial system for alerting the host to the presence of numerous infectious agents. Infectious microbes display certain molecular patterns that are necessary for microbial virulence. Many of these molecular patterns such as lipopolysaccharide in the outer membranes of gram negative bacteria seem to be particularly potent activators of mammalian cells. The mammalian receptors responsible for recognition of PAMPs are called pattern recognition receptors. The failure of the immune system to recognize a pathogen’s PAMP could lead to a delay or blunting of the immune response, resulting in unchecked invasion by the microbe.