Macrophages and related cells employ a different mechanism to recognize the pathogen. Their receptors recognize certain motifs present on the invading pathogen that are very unlikely to be present on a host cell. Such repeating motifs are recognized by pattern recognition receptors (PRRs) like the toll-like receptors (TLRs) expressed by the macrophages. Since the same receptor could bind to a given motif present on surfaces of widely disparate microorganisms, this mode of recognition is relatively nonspecific, and constitutes an innate immune response.