Читать книгу Immunology - Richard Coico - Страница 60

Innate immunity refers to immune mechanisms that are present from birth and serve as the first line of defense against microbes and other offending pathogens. Responses occur within minutes of exposure to invading pathogens. Every living organism is confronted by continual intrusions from its environment and sometimes, such intrusions involve pathogens which, in the absence of host defense mechanisms, would otherwise take advantage of our bodies for their own survival. Early defense provided by the innate immune system is therefore critical to survival of the host. These defenses range from physical and chemical barriers—elements of innate immunity—to highly sophisticated systems that constitute the adaptive immune system. In this chapter, we describe the principal elements of innate immunity. We will discuss the participating organs, cells, and molecular components of innate immunity and their physiological roles that, in many cases, include dynamic interactions with elements of the adaptive immune system. Thus, innate immune responses are important not only because they are an independent arm of the immune system but also because they profoundly influence the nature of adaptive immune responses.

The innate immune system is a phylogenetically ancient defense system which appeared during evolution of multicellular organisms ~750 million years ago, in contrast with adaptive immune mechanisms which appeared ~350–500 years ago in vertebrates. During its long evolutionary history, the system co‐evolved with microbes to protect multicellular organisms from infection. It is noteworthy that several components of the mammalian innate immune system are remarkably similar to those found in plants and insects, suggesting a common genetic ancestry. Examples include structurally similar peptides called defensins which are toxic to bacteria and fungi. Another example is a family of receptors called Toll‐like receptors (TLRs) that will be discussed in detail later in this chapter. TLRs recognize pathogenic microbes by virtue of their ability to bind to structural shapes or molecular patterns expressed on whole groups of pathogens but not the host. Hence, TLRs are said to provide defense against pathogens through pattern recognition. Interactions between innate immune cells expressing TLRs and pathogens expressing pathogen‐associated molecular patterns triggers a cascade of host cell intracellular events leading to selected gene expression and cell activation that, in many instances, ends with destruction of the invading pathogen. In other instances, innate immune mechanisms, while partially protective, are not fully capable of preventing an ensuing infection. Fortunately, the adaptive immune system is then mobilized over time (days to weeks) to generate a pathogen‐specific response that has the advantage of establishing long‐term memory responses to help prevent future infection caused by invading pathogens.