Physico-chemical barriers of innate immunity

July 23, 2018

Introduction

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.

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