Physico-chemical barriers of innate immunity


  • Innate immunity is also known as native immunity.
  • It is a resistance with which a person or lower animal is born and is non-specific.
  • This type of immunity is present throughout our life.
  • It may be of various types like species immunity, racial immunity or individual immunity.
  • Various factors like age, hormones, nutrition, etc. influence the innate immunity in the host.
  • Natural or innate immunity has three components. Among them physiochemical barriers are also one of them.

Physiochemical barriers

  • They are the epithelial surface like skin and mucosae, and cilia while bactericidal secretions behave as chemical barriers.
  • The animal body is a close system separated from the environment by skin and mucous membranes and they are impermeable to the particulate material of the size of the bacteria.

a) Skin

  • The skin acts as a mechanical barrier to the invading microorganisms and also provides bactericidal secretions.
  • The sebaceous secretions containing unsaturated fatty acids (oleic acid) and free saturated fatty acids have bactericidal and fungicidal
  • The dry skin with high salt concentration in drying sweat inhibits or is lethal to many bacteria and fungi.
  • The skin can be freed of transient flora easily but the resident flora cannot be removed even by washing or by the use of disinfectant.
  • The superficial microorganisms of the resident flora may be diminished by vigorous surgical “scrubbing” but they get replenished rapidly from sebaceous and sweat glands.

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b) Nose, naso-pharynx and respiratory tract

  • The moist surfaces of the mucous membrane lining of the nasal passage arrest various inhaled bacteria and other particulate material.
  • Thus, the inspired air is largely freed of bacteria in the upper respiratory passage.
  • Though some of the organisms skip this passage, they get trapped in the bronchial mucosa in the larynx and only a few may reach the bronchioles and alveoli.
  • The sticky mucous secretions of respiratory mucosa and the hair like cilia sweep the secretions containing the foreign particulate.
  • They are swept towards the oropharynx where it is swallowed or is coughed out.
  • The cough reflex plays an important role of respiratory tract.
  • The small particulate materials which manage to reach the alveoli are ingested by the phagocytic cells present there.
  • The nasal and respiratory secretions contain muco-polysaccharides which can neutralize influenza and some other viruses.
  • Inherited or acquired defect in the function of respiratory cilia or mucous or both make the lungs vulnerable to infection.

c) Mouth, stomach and intestinal tract

  • Saliva possesses mild bactericidal action.
  • The anaerobic colon bacteria produce fatty acids with antibacterial activity.
  • Colonization resistance is offered by the predominant normal flora of the intestine.
  • The intestinal anaerobic micro flora prevents the super infection by coliforms during antibiotic therapy.
  • The low pH of gastric acid destroys most of the ingested bacteria.

d) Conjunctivae

  • The conjunctiva is freed of bacteria and dust particles due to the activity of tears.
  • Tears flush the eyes which contain lysozyme which is bactericidal in action.

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e) The genitourinary tract

  • The flushing action of the slightly acidic urine maintains sterility of both male and female urethra.
  • Semen is believed to contain antibacterial substances.
  • The acid reaction of the vaginal secretion in female due to fermentation of glycogen by Lactobacillus acidophilus (normal flora) is markedly bactericidal towards most pathogenic microorganisms.




Physico-chemical barriers of innate immunity