Barrier Defenses and the Innate Immune Response

Core Concepts

First line of defense: physical and chemical barriers

The skin is the largest barrier: the epidermis is a multilayered sheet of keratin-filled cells that is tough, dry, and constantly shedding, making it hard for microbes to attach or penetrate. Where skin is absent — the respiratory, digestive, urinary, and reproductive tracts — mucous membranes take over: epithelial cells secrete mucus, a sticky fluid that traps microbes, while cilia in the airways sweep the mucus upward to be swallowed or coughed out. Chemical defenses reinforce these surfaces: tears and saliva contain lysozyme, an enzyme that breaks down bacterial cell walls; defensins are antimicrobial peptides produced by epithelial cells and phagocytes; gastric acid and digestive enzymes destroy most swallowed organisms; and the normal microbiota outcompete pathogens for space and nutrients on the skin and in the gut. Sebum and other secretions also create an environment hostile to many microbes.

Second line of defense: phagocytes

When a pathogen breaches the barriers, phagocytosis begins. The main phagocytes are neutrophils, the most abundant white blood cells and the first responders at an infection site, and macrophages, larger, longer-lived cells that are stationed in tissues (where they are called resident macrophages). Phagocytosis proceeds in steps: the phagocyte is attracted to the site by chemical signals (chemotaxis), binds the target, engulfs it into a vesicle (phagosome), fuses that vesicle with a lysosome to form a phagolysosome, and kills and digests the microbe with enzymes and reactive oxygen species. Recognition is made easier by opsonization: antibodies or complement proteins coat the pathogen, and phagocytes bind these tags with special receptors — "seasoning" the target so it is easier to eat.

Natural killer cells and antimicrobial proteins

Natural killer (NK) cells are lymphocytes that patrol the blood and tissues looking for cells in trouble. They kill virus-infected cells and tumor cells by releasing granules containing perforin (which punches pores in the target's membrane) and granzymes (which enter through those pores and trigger apoptosis). NK cells act without prior sensitization — they recognize infected cells partly by detecting missing or altered "self" markers (MHC class I molecules), a useful trick because many viruses and cancers downregulate those markers. Among the antimicrobial proteins, interferons are cytokines released by virus-infected cells that signal neighboring cells to produce antiviral proteins, limiting viral spread, and that activate NK cells and macrophages. The complement system is a cascade of ~30 plasma proteins that, once activated, opsonizes pathogens, recruits phagocytes, and can form a membrane attack complex that lyses the pathogen — complement also links directly to the adaptive response because antibodies can activate it.

Inflammation and fever

Inflammation is the local response to tissue damage or infection. Injured cells and immune cells release chemical mediators (such as histamine from mast cells) that produce the classic signs: redness and heat (vasodilation increases blood flow), swelling (increased vascular permeability lets fluid and proteins leak into the tissue), pain (mediators sensitize pain receptors, and swelling compresses nerves), and loss of function. Vasodilation and permeability changes also bring phagocytes to the site; leukocytes adhere to the vessel wall, squeeze between endothelial cells (emigration/diapedesis), and move toward the source by chemotaxis. Inflammation is protective — it contains the infection, dilutes toxins, and delivers immune cells — but excessive or chronic inflammation damages tissue. Fever is a systemic response: pyrogens (including cytokines such as IL-1) reset the hypothalamic thermostat upward, raising core temperature, which can inhibit some pathogens and enhance immune activity. Fever is a defense mechanism, not a disease; whether and how to treat it is a clinical judgment.

Innate versus adaptive at a glance

Innate immunity is fast (minutes to hours), non-specific, uses broad pattern recognition, and has no memory. Adaptive immunity (Topics 3–4) is slower on first exposure (days), highly specific, remembers prior encounters, and produces antibodies and memory cells. The two arms cooperate: innate responders present antigen to and activate adaptive cells, and adaptive products (antibodies, complement-binding) make innate effectors more effective.

Common Confusions