Innate Nonspecific Host Defenses

Despite relatively constant exposure to pathogenic microbes in the environment, humans do not generally suffer from constant infection or disease. Under most circumstances, the body is able to defend itself from the threat of infection thanks to a complex immune system designed to repel, kill, and expel disease-causing invaders. Immunity as a whole can be described as two interrelated parts: nonspecific innate immunity, which is the subject of this chapter, and specific adaptive host defenses, which are discussed in the next chapter.

The nonspecific innate immune response provides a first line of defense that can often prevent infections from gaining a solid foothold in the body. These defenses are described as nonspecific because they do not target any specific pathogen; rather, they defend against a wide range of potential pathogens. They are called innate because they are built-in mechanisms of the human organism. Unlike the specific adaptive defenses, they are not acquired over time and they have no “memory” (they do not improve after repeated exposures to specific pathogens).

Broadly speaking, nonspecific innate defenses provide an immediate (or very rapid) response against potential pathogens. However, these responses are neither perfect nor impenetrable. They can be circumvented by pathogens on occasion, and sometimes they can even cause damage to the body, contributing to the signs and symptoms of infection (the photo above).

Sections

• Physical Defenses — the physical barriers (the skin, mucous membranes, and endothelia) and mechanical defenses that block pathogens from entering the body, and the role of the microbiome as a first-line defense against infection and disease.
• Chemical Defenses — the chemical and enzymatic mediators found in body fluids, antimicrobial peptides, plasma protein mediators and acute-phase proteins, the classic, alternate, and lectin complement pathways, cytokines, and inflammation-eliciting mediators.
• Cellular Defenses — the components of blood and the process of hematopoiesis, and the characteristics and innate-immune functions of the granulocytes (neutrophils, eosinophils, basophils, and mast cells) and agranulocytes (natural killer cells and monocytes) found in peripheral blood.
• Pathogen Recognition and Phagocytosis — how leukocytes migrate from peripheral blood into infected tissue by extravasation, the mechanisms by which leukocytes recognize pathogens, and the process of phagocytosis by which phagocytes destroy and degrade pathogens.
• Inflammation and Fever — the signs of acute and chronic inflammation and fever and why they occur, and the advantages and risks posed by the inflammatory response.

This chapter is adapted from Microbiology, Chapter 17: Innate Nonspecific Host Defenses by Nina Parker, Mark Schneegurt, Anh-Hue Thi Tu, Philip Lister, Brian M. Forster, and OpenStax, © OpenStax, licensed under CC BY-NC-SA 4.0. Access the original for free at openstax.org. Each section page records its own changes from the source. Changes: the chapter-opening figure is the source’s photo, re-encoded for the web, with its alt text rewritten to describe what is actually visible; the source’s in-text figure reference is rendered as “the photo above”; the module marks “nonspecific innate immunity” as a term (class no-emphasis, so the source itself gives it no visual styling), which is rendered here as plain prose; the module has no footnote.