Innate immunity

Innate immunity — also called non-specific or natural immunity — is the resistance to infection that an individual is born with, rather than resistance built up through prior exposure to a particular pathogen. It is described as non-specific because the same defences act broadly against a wide range of potentially infectious agents rather than targeting one particular antigen.

The chapter organizes innate defences into four groups. Anatomical barriers are physical surfaces: the skin blocks microbial entry outright and its mildly acidic surface (pH 3–5) further discourages microbial growth, while mucus membranes physically trap microorganisms and out-compete them for attachment sites. Physiological barriers work through the body's internal chemistry and conditions — normal body temperature and fever both slow pathogen growth, the strong acidity of gastric HCl kills most swallowed microbes, and chemical mediators such as lysozyme (which cleaves bacterial cell walls), interferons (which induce an antiviral state in uninfected neighbouring cells) and complement-derived substances (which lyse pathogens or mark them for engulfment) all act directly on invaders. Phagocytic barriers are specialised cells — monocytes, neutrophils and tissue macrophages — that engulf and digest whole microorganisms. Inflammatory barriers com …