The innate immune system is an evolutionarily ancient form of host defense
found in most multicellular organisms. Inducible responses of the innate im
mune system are triggered upon pathogen recognition by a set of pattern rec
ognition receptors. These receptors recognize conserved molecular patterns
shared by large groups of microorganisms. Recognition of these patterns all
ows the innate immune system not only to detect the presence of an infectio
us microbe, but also to determine the type of the infecting pathogen. Patte
rn recognition receptors activate conserved host defense signaling pathways
that control the expression of a variety of immune response genes.