Earthworms belonging to oligochaete annelids became an important model for comparative immunologists in the early 1960s with the publication of transplantation experimental results reporting that autologous transplants of earthworm body wall pieces were accepted, but xenograft or even allografts were not. These transplantation experiments proved the existence of self or nonself recognition in earthworms, paving the way for extensive studies on the earthworm immune mechanisms that evolved to prevent the invasion of pathogens. In 1989 Charles Janeway, Jr., published a general concept based on the existence of pattern recognition receptors (PRRs) on the immune cells that recognize and bind conserved molecular structures of microorganisms known as pathogen-associated molecular patterns and thus trigger the immune response to potential pathogens. In the present review, the structure, specificity, and expression profile of PRRs characterized in earthworms are discussed and their role in innate defense suggested.