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Innate immune system

Last updated: August 27, 2019


The innate immune system provides a non-specific first line of defense against pathogens. It comprises physical barriers (e.g. the skin) and both cellular (granulocytes, natural killer cells) and humoral (complement system) defense mechanisms. The reaction of the innate immune system is immediate, but unlike the adaptive immune system, it does not provide permanent immunity against pathogens.



The innate immune response is composed of physical, chemical, cellular, and humoral defense mechanisms against pathogens. It is present at birth and does not require imprinting or adaptation to specific antigens. For this reason, it is also referred to as nonspecific immunity. Response to pathogens is rapid, occurring within minutes to hours of exposure. The components of the innate immune system include neutrophils, macrophages, monocytes, dendritic cells, natural killer cells, and the complement system.

Physical, chemical, and biological mechanisms


The cells involved in innate immunity are listed below. For the specific function of the individual populations see basics of hematology.

Cellular mechanisms

Respiratory burst (oxidative burst)

Defects in the NADPH oxidase cause chronic granulomatous disease.

Myeloperoxidase contains a blue-green heme-containing pigment that gives sputum and pus a green-yellow color.

Catalase positive organisms can degrade H2O2 into H2O and O2 and prevent the formation of hydroxyl-halide radicals.

HLA system and pattern recognition receptors


Humoral mechanisms

Humoral mechanisms

The humoral mechanisms of innate immunity is mediated by proteins that are secreted into bodily fluids or the blood stream. These proteins often initiate additional immune responses via:

Acute phase proteins

Complement system

Defects of innate immunity

Immune deficiency Examples of causes

Increased susceptibility to infection for

Granulocyte defect

Phagosome defect


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