Recognition Of Microbes And Damaged Self By The Innate Immune System

The specificities of innate immune recognition have evolved to combat microbes and are different from the specificities of the adaptive immune system in several respects (Table 4-1).

The innate immune system recognizes molecular structures that are characteristic of microbial pathogens but not mammalian cells. The microbial substances that stimulate innate immunity are called pathogen-associated molecular patterns (PAMPs). Different classes of microbes (e.g., viruses, gram-negative bacteria, grampositive bacteria, fungi) express different PAMPs. These structures include nucleic acids that are unique to microbes, such as double-stranded RNA found in replicating viruses and unmethylated CpG DNA sequences found in bacteria; features of proteins that are found in microbes, such as initiation by N-formylmethionine, which is typical of bacterial proteins; and complex lipids and carbohydrates that are synthesized by microbes but not by mammalian cells, such as lipopolysaccharide (LPS) in gram-negative bacteria, lipoteichoic acid in grampositive bacteria, and mannose-rich oligosaccharides found in microbial but not in mammalian glycoproteins (Table 4-2). In actuality, there are only a limited number of fundamental differences between microbial molecules and the molecules that higher organisms produce. Thus, the innate immune system has evolved to recognize only a limited number of molecules, most of which are unique to microbes, whereas the adaptive immune system is capable of recognizing a much wider array of foreign substances whether or not they are products of microbes.

The innate immune system recognizes microbial products that are often essential for survival of the microbes.

This feature of innate immune recognition is important because it ensures that the targets of innate immunity cannot be discarded by microbes in an effort to evade recognition by the host. An example of a target of innate

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