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.

Adaptive immunity is based on receptors that are generated by somatic mechanisms during the ontogeny of each individual organism. These mechanisms generate a diverse repertoire of antigen receptors with random specificities, which are clonally distributed on two types of lymphocytes: T cells and B cells. Consequently, the specificity of the receptors expressed on each lymphocyte is not predetermined, and neither is the response that can be induced in lymphocytes upon ligation of their receptors by antigen. However, induction of an immune response is only appropriate if the antigen recognized is derived from, or belongs to, a pathogen. Activation of lymphocytes specific for self antigens, or innocuous persistent environmental antigens, may result in autoimmune disorders and deleterious hypersensitivity reactions, respectively. Moreover, protection from different groups of pathogens may require induction of quite different types of effector responses. Again, activation of inappropriate effector responses that do not provide protection against the relevant pathogen often leads to various immunopathologies. Therefore, the adaptive immune response requires signals that provide information about the origin of the antigen and the type of response to be induced. Evidence has accumulated over the past few years to suggest that these signals are provided by the innate immune system (for review, see