Innate immunity: the virtues of a nonclonal system of recognition

The immune system evolved under selective pressure imposed by infectious microorganisms. As a result, all multicellular organisms have developed various defense mechanisms that have the capacity to be triggered by infection and to protect the host organism by destroying the invading microbes and neutralizing their virulence factors. These phylogenetically ancient defense mechanisms, also known as the innate immune system, use germline-encoded receptors for the recognition of microbial pathogens. This feature distinguishes the innate immune system from the other component of immunity, the adaptive immune system, found only in vertebrates.