Types of Immunity > Innate Immunity

Innate immunity is the inborn, non-specific defence that protects the body from pathogens from birth.

• Innate immunity is a non-specific defence present at birth.
• It is a natural or hereditary resistance of the body.
• It is not affected by prior infection or immunisation.
• It protects the body mainly by preventing the entry of foreign agents and by destroying them after entry.

Innate immunity includes four types of barriers: physical, physiological, cellular, and cytokine. These barriers serve as the body's first line of defence against infection.

1. Physical barriers:

• Skin acts as the primary protective barrier, preventing the entry of microorganisms.
• Mucus covering the epithelium of the respiratory, gastrointestinal, and urogenital tracts helps trap microbes.
• Hair near the nasal openings helps arrest inhaled particles.
• Particles caught in mucus are swept towards the pharynx and are swallowed or coughed out.
• The cough reflex is an important protective mechanism of the respiratory tract.
• The flushing action of urine removes bacteria from the urethra, and lachrymal secretions help remove foreign particles from the conjunctiva.

2. Physiological barriers:

• Acid in the stomach destroys many bacteria.
• Saliva has an inhibitory effect on microorganisms.
• Tears help prevent microbial growth.
• Tears contain lysozyme, an antibacterial substance active against some pathogenic Gram-positive bacteria.
• Sebaceous secretions, long-chain fatty acids, and the salt present in drying sweat also show protective activity on the skin.
• Spermine and zinc present in semen are antibacterial.

3. Cellular barriers:

• Cellular defence is carried out mainly by phagocytic cells that ingest and destroy foreign particles and micro-organisms.
• These include polymorphonuclear leukocytes (neutrophils), monocytes, and macrophages.
• Phagocytic cells are grouped as microphages and macrophages.
• Natural killer cells are important in non-specific defence against viral infections and tumours.

4. Cytokine barriers:

• Virus-infected cells secrete proteins called interferons.
• Interferons protect nearby non-infected cells from further viral infection.
• Certain white blood cells also release interferons.

Other protective factors:

• The complement system comprises more than 30 serum proteins that circulate in inactive form in the blood.
• In the presence of microbial pathogens, the complement cascade gets activated and helps eliminate pathogens.
• During infection or injury, the concentration of acute-phase proteins increases.
• These include C-reactive protein, mannose-binding protein, alpha-1-glycoprotein, and serum amyloid P.
• These proteins help enhance host resistance, prevent tissue injury, and promote repair of inflammatory lesions.
• Fever is a natural defence response after infection.
• It helps accelerate physiological processes that destroy invading pathogens and stimulates interferon production.

• Innate immunity is a non-specific, natural defence present at birth that prevents the entry of and destroys foreign agents.
• Physical barriers, such as the skin and mucous membranes, physically block and trap invading microorganisms.
• Physiological barriers utilise bodily secretions - like stomach acid, saliva, and lysozyme in tears - to inhibit or destroy microbial growth.
• Cellular barriers rely on phagocytic cells (neutrophils, macrophages) and Natural Killer cells to actively ingest and destroy pathogens.
• Cytokine barriers consist of interferons, which are secreted by virus-infected cells to protect surrounding healthy cells from further viral infection.
• Additional systemic defences include fever, the complement protein system, and acute-phase proteins that enhance host resistance and eliminate pathogens.