Innate (Inborn) Immunity

i. Innate immunity or Inborn immunity : Innate immunity (also called inborn immunity) is the resistance to infection an individual is simply born with, by virtue of their genetic make-up. It is present from birth, is not improved or altered by prior contact with a microorganism or by immunization, and can be either non-specific (a general degree of resistance to infection) or, in some forms, directed at a particular kind of pathogen. Innate immunity is built from several complementary layers of defence:

• Epithelial (barrier) surfaces: Intact skin and the mucous membranes lining the body's openings are the first physical barrier against microbial invasion. Healthy skin is bactericidal because of the high salt concentration in drying sweat, and sebaceous secretions with long-chain fatty acids kill bacteria and fungi. In the respiratory tract, nasal hairs trap inhaled particles, and mucus lining the epithelium traps the rest and sweeps them back to the pharynx to be swallowed or coughed out — the cough reflex itself is an important respiratory defence. Saliva constantly bathes the mouth and inhibits microorganisms; stomach acid destroys swallowed microbes; the flushing action of urine clears bacteria from the urethra; spermine and zinc in semen are antibacterial; and lachrymal (tear) secretions flush the conjunctiva and contain the antibacterial enzyme lysozyme (first described by A. Fleming), which is especially active against some Gram-positive bacteria.
• Antimicrobial substances in blood and tissues: The complement system — over 30 inactive serum proteins normally circulating in the blood — is activated ('the complement cascade') when it detects microbial presence, helping to eliminate pathogens. Interferons are cytokines released by virus-infected cells and certain white blood cells that signal neighbouring cells to defend themselves against the same viral infection.
• Cellular factors: Phagocytic cells (discovered by Metchnikoff in 1882), grouped as microphages and macrophages, ingest and destroy invading microorganisms and other foreign particles in the blood and tissues. A class of lymphocytes called Natural Killer (NK) cells provides non-specific defence particularly against viral infections and tumour cells.
• Fever: A rise in body temperature following infection is itself a defence mechanism — it speeds up physiological processes that help destroy invading pathogens, and it stimulates interferon production, aiding recovery from viral infections. …