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Innate immunity is the body’s natural inborn resistance to infection and it is quick in responding to invasive pathogenic microbes or antigens that enters the body. It is a component of the immune system that is an inherited protective mechanism, and which protect the body of an animal or human host from many kinds of pathogens. Innate immunity is a nonspecific immune response to antigens, and it is the first line of defense that is not acquired through prior contact with an infectious disease or pathogen. This form of immunity is not directed at any specific organism or antigen, instead they are intended for all pathogens or foreign substances to which the body is exposed. The response of the innate immune system to pathogen is static. This implies that it does not improve with repeated exposure to antigens and there is no immunological memory on subsequent exposures. Subsequent exposure of the body to antigens does not change or increase the level of response of this type of immunity.


Innate immunity comprises four (4) types of defensive barriers which help to keep away pathogenic microorganisms from the host body, and these are:

  • Physiologic barriers such as body temperature, pH level, chemical mediators (e.g., cytokines, lysozymes, antimicrobial peptides, interleukins, surfactants, lipids, enzymes, interferon, and complements).
  • Phagocytic barriers which include special forms of the white blood cells (e.g., monocytes, neutrophils, and eosinophils) which internalize/engulf and digest or breakdown foreign substances including whole microorganisms that enter the body.
  • An inflammatory barrier which occurs following tissue damage. Inflammatory barriers are usually characterized by redness, swelling (oedema), pain and heat. In this type of innate immunity, vascular fluids containing protective serum proteins and phagocytic cells are released to the site of infection to stop the spread of the infection.

In healthy individuals, most of the microorganisms encountered by the body of a human host are readily cleared and eliminated by the mechanisms of the innate (nonspecific) immune system even before they activate the adaptive immune response to the invading foreign substance. Innate immune response is very vital as the first line of defense against an infection because it provides the very primary defense during the critical stage of infection (i.e., at the onset of host exposure to an antigen) since the adaptive (specific) immune response takes some time to come up. However, there are some hosts factors that can affect and cause variations in the response of innate immunity based on individual differences such as: age, sex, nutrition, host genetic determinants, stress, hormonal activity, and fatigue.

Some diseases or infections that occur in humans are sex-specific. Malnutrition also predisposes the body to infection because undernourishment affects the immune system and weakens its ability to protect the body from foreign substances or immunogens. Newborns and the elderly are more prone to some infections due to the low level of their immune system which is either still developing (as in infants) or is in a declining stage (as in old people). Stress, fatigue, hormonal imbalance and the genetic makeup of individuals are other predisposing factors that may affect the level of response of the immune system of a human host due to differences in their physiological and environmental conditions.      

Further reading

William E.P (2003). Fundamental Immunology. 5th edition. Lippincott Williams and Wilkins Publishers, USA.

Stevens, Christine Dorresteyn (2010). Clinical immunology and serology. Third edition. F.A. Davis Company, Philadelphia.

Silverstein A.M (1999). The history of immunology. In Paul, WE (ed): Fundamental Immunology, 4th edition. Lippincott Williams and Wilkins, Philadelphia, USA.

Paul W.E (2014). Fundamental Immunology. Seventh edition. Lippincott Williams and Wilkins, USA.

Male D, Brostoff J, Roth D.B and Roitt I (2014). Immunology. Eight edition. Elsevier Saunders, USA.

Levinson W (2010). Review of Medical Microbiology and Immunology. Twelfth edition. The McGraw-Hill Companies, USA.

Berzofsky J.A and Berkower J.J (1999). Immunogenicity and antigen structure. In Fundamental Immunology, 4th edition., W.E. Paul, ed., Lippincott-Raven, Philadelphia. 

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