Innate immune system

The **innate immune system** defends the host from infections.  It includes cells which recognize and respond to pathogens (germs) right away. The innate immune system response is not specific: it responds the same way to all pathogens that it recognises.

Unlike the adaptive immune system, the innate immune system does not give long-lasting immunity against specific infections.

Innate immune systems rapidly defend against infections in all plant and animal life. The system is not adaptable and does not change over the course of an individual's lifetime.

The vertebrate innate immune system:
*Acts as a physical and chemical barrier to infectious agents. If that fails, then:
* Gets  immune cells (such as natural killer cells) to the infection by producing cytokines.
* Starts the *complement cascade* to identify bacteria, activate cells and clear out dead cells.
* White blood cells identify and remove foreign substances present in organs, tissues, blood and lymph.
* Activates the adaptive immune system through a process known as *antigen presentation*.

## Anatomical barriers 
The innate immune system includes the skin.  The outer layers of the skin are called "epithelial". Epithelial cells form a waxy physical barrier that keeps out most infectious agents. These cells are the innate immune system's first line of defense against invading organisms.

Old skin cells drop off, and this helps remove bacteria that have stuck to the skin.

Chemical factors produced during inflammation attract phagocytes, especially neutrophils.

## Complement system 
The complement system is a biochemical cascade of the immune system that helps antibodies clear pathogens or mark them for destruction by other cells.

The cascade is composed of many plasma proteins, which are made in the liver.  The proteins work together to:
* trigger the recruitment of inflammatory cells.
* tag pathogens for destruction by coating their surface.
* disrupt the plasma membrane of an infected cell, causing cytolysis of the infected cell, and death of the pathogen.
* rid the body of neutralized antigen-antibody complexes.

Elements of the complement cascade can be found in many non-mammalian species including plants, birds, fish and some species of invertebrates.

## Cells of the innate immune response
 image of normal circulating human blood. One can see red blood cells, several knobby white blood cells including lymphocytes, a monocyte, a neutrophil, and many small disc-shaped platelets.]]

All white blood cells (WBC) are known as leukocytes. Leukocytes are different from other cells of the body: they work like independent, single-celled organisms. They can move freely, and capture cell debris, foreign particles, or invading microorganisms. They are produced by blood-forming stem cells in the bone marrow.

### Mast cells 
Mast cells are a type of innate immune cell in connective tissue and the mucous membranes. They are intimately associated with defence against pathogens and wound healing. They are also often associated with allergy and anaphylaxis.

Marine fish sources have high levels of antimicrobial compounds. Testing with live fish showed that fish peptides used in food/feed ingredients worked well.

## References