What is Natural immunity?

Natural immunity is also known as innate immunity or non specific immunity. It is a pre-existing and natural defense mechanism inherited from parents to offspring. It is different in different species, races and even in different individuals of the same species.

Generally it depends on various other factors such as age, physical fitness, nutrition, social condition etc. also. For example infants and old peoples are more susceptible to diseases than the remaining age groups.

Since it is natural and pre-existing it is not specific for any particular pathogen. The non specific immunity provides protection through different types of natural barriers such as anatomical, mechanical, physiological, phagocytic, inflammatory etc.

Since anatomical and mechanical barriers are preventing the entry of pathogens, protection provided by these barriers is considered as the “first line of defense”.

Defense provided through enzymes and phagocytes following the entry of pathogen are referred as second line of defense and the protection in the later stages through inflammation involving natural killer cells, mast cells etc. is considered as third line of defense.

First Line of Defense:

It is the protection provided by anatomical and mechanical barriers. Skin, surface mucosal linings of gastro intestinal track, urinogenital track, and respiratory track are considered as anatomical barriers.

Mechanical actions such as coughing, sneezing, peristaltic movements etc. are the mechanical barriers that obstruct the pathogens entry in to the body.

Skin:

Skin The impermeable nature of skin with epidermis and dermis and low PH (3 to 5) due to sebum secretions from the sebaceous glands prevents entry and growth of most micro organisms. If pathogens managed to enter into the body, they are removed by the circulating immune cells in the blood vessels of dermis.

Mucus membrane:

The mucosal lining of respiratory, urinogenital, gastrointestinal etc. tracts open to the external environment prevents adherence of microbes to the epithelial layers by trapping them in their sticky substance – mucus. The movements of cilia present on epithelial membrane of respiratory track helps in the removal of microbes trapped in mucus.

Mechanical actions: Mechanical actions like coughing, sneezing, peristaltic movements etc. help in driving out the foreign agents from respiratory and digestive tracks of animals.

Since all living beings follow the law of evolution “survival of the fittest” some microbes acquired adaptations to cross over the defensive barriers. They adhere to the mucosal layer either with their modified body structures or surface characters.

For example in case of the bacterium Neisseria gonorrhoeae the pathogenic mechanism is involved in the attachment of the bacterium to nonciliated epithelial cells via pili (fimbriae) and the production of lipopolysaccharide endotoxin.

The fimbriae, which play a major role in adherence, extend several micrometers from the cell surface. The lipopolysaccharide produced by Neisseria meningitidis is highly toxic, and it has an additional virulence factor in the form of its antiphagocytic capsule.

Virus particles attaches to their host cells through surface particles often called as viral attachment proteins. In enveloped viruses the surface glycoproteins oligomerizes to form spikes that protrude from the surface and help in their attachment to host cells.

Second Line of Defense

The action of certain physiological and biochemical barriers like temperature, pH etc., substance such as enzymes of complement system, natural antibodies e.g. blood group antibodies, interleukins and phagocytic cells present in body fluids (humus) provide second line of defense.

An alteration in the body temperature affects the growth rate of microbes. For example high body temperature in birds prevents growth of the bacteria that can cause anthrax. Hence raise in temperature of the body during infection is an attempt to inhibit the growth of certain infection causing agents.

PH of the surroundings also influences growth of microbes. For example presence of microbes is almost nil in the stomach region due to low PH in the presence of gastric acid – HCL. Occurrence of fungal infections in foot is due to absence of sebaceous glands that can create low PH environment.

Hydrolytic enzymes found in saliva, tears, breast milk can kill microbes and work as anti microbial agents and prevent entry and multiplication of microbes into the body. Enzymes of complement system, and interleukins secreted by different immune cells further help in killing and removal of pathogens.

Phagocytosis is one of the defense weapons used by the immune system to remove the pathogens entered in to the body successfully.

A group of immune cells such as Neutrophiles, basophils, eosinophils, macrophages, monocytes, natural killer cells etc. phagocytise the pathogens. The phagocytic cells reach the foreign agent with the help of chemotaxis.

Third Line of Defense:

Third line of defense involves various cells of the immune system. Immune response to small wounds or injuries or mild infections involving various immune cells is routinely carried in all individuals in the same way.

Different cells of immune system move to the region of infection or damage through chemotactic action to remove pathogens and damaged tissue.

The whole procedure of immune response is known as “inflammation”. Vasodilatation, marginalization, erythema, edema, and diapedesis are some of the common reactions that take place in the body during inflammation.