Innate immune cells possess various classes of membrane receptors, termed "Pattern Recognition Receptors",
The receptors bind to molecules such as LPS, dsRNA, or other generic molecules which are produced only by microbes.
Several families of Pattern Recognition Receptors proteins exist but the prototypical family is known as the "Toll-like Receptor" family.
Pattern Recognition Receptors not only allow innate immune cells to recognize that a microbial infection is present, but depending on which receptors are activated allow innate immune cells to roughly determine what type of infection is occurring (i.e. bacterial, viral, fungal, etc.).
Recognizing the basic category of infection assists the innate immune response in shapaing the later adaptive immune response in the most effective way.