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Innate immune system
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 give immediate defence against infection, and are found in all plant and animal life. The innate system is the evolutionarily older defense strategy. It is the main immune system found in plants, fungi, insects, and in primitive multicellular organisms. 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:
  • 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.

The skin continues internally as the lining of the intestine and lung. In the intestines or lungs, movement by peristalsis or cilia helps to remove infectious agents. Also, mucus traps infectious agents. In the intestines, gut flora can prevent pathogenic bacteria by secreting toxic substances, or by competing with pathogenic bacteria for nutrients or for attachment to cell surfaces.

The flushing action of tears and saliva helps prevent infection of the eyes and mouth.


Inflammation is one of the first responses of the immune system to pathogens or foreign substances that get past the anatomical barriers.

Inflammation is stimulated by chemical factors released by injured cells. It sets up a physical barrier against the spread of infection, and promotes healing of damaged tissue after the clearance of pathogens.

Chemical factors produced during inflammation attract phagocytes, especially neutrophils. Neutrophils then trigger other parts of the immune system.

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.

Invertebrate immune systems

Antimicrobial peptides

Various AMPs
Various structures of antimicrobial peptides

Antimicrobial peptides, or host defence peptides, are part of the innate immune response. They are found among all classes of life. These peptides are potent, broad spectrum antibiotics. They kill both gram negative and gram positive bacteria, mycobacteria (including Mycobacterium tuberculosis), enveloped viruses, fungi and even transformed or cancerous cells.

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

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