Insects are exposed to microorganisms (bacteria, virusses and fungi) and parasites on a regular basis. To survive in a world full of microorganisms and parasites, insects developed a potent defense mechanism that recognizes and removes microbial threats. Insects depend on innate immunity for their survival. This innate immunity can be divided into two classes, the cellular response and the systemic response.




The cellular response

Much like we do, insects also have blood. But quit unlike us, insects do not have a circulatory system in which the hearth circulates the blood. Insects have a body cavity (the hemocoel) in which the blood resides (insect blood is called hemolymph) and a tube-like hearth which sucks the hemolymph from one side and pumps it back into the hemocoel on the other side (see 14 in the figure below). Generally insects do not use hemolymph (blood) to transport oxygen but it is very important in the immune response. The hemolymph consists of 3 cell types:

1) plasmatocytes (comparable to our white bloodcells)
90-95% of the hemolymph

2) crystal cells (storage of antimicrobial particles)
5% of the hemolymph

3) lamellocytes (can handle larger microorganisms than plasmatocytes)
very little

 

Anatomie insect
Piotr Jaworski, PioM; 17 V 2005r., POLAND/Poznań, Insect scheme; license: CC Attribution-ShareAlike 2.0 (http://creativecommons.org/licenses/by-sa/2.0/) {{cc-by-sa-2.0}}

 

The crystal cells contain particles that are lethal for microorganisms and are released upon infection. The plasmatocytes and lamellocytes are cells that eat microorganisms in a process which is called phagocytosis. The plasmatocyte surrounds for example a bacterium and consumes it. The bacterium will be on the inside of the plasmatocyte and will be broken down there. (see movie below)

The systemic immune response

Next to the cellular immune response, insects synthesize massive amounts of antimicrobial peptides (AMPs) in response to infection. The synthesis of AMPs in response to infection is called the systemic immune response. These AMPs can be produced in the liver (called fatbody in insects). The liver produces large amounts of AMPs which it secretes into the hemolymph where the AMPs fight the infection. But to make these AMPs, first the infection needs to be recognized, the recognition of infection induces a chain reaction which will eventually lead to the production of AMPs. This chain reaction (or signal transduction) works because the different subunits of this chain are already present. I will not talk about all the specific proteins involved in this chain but I will explain the general mechanisms. First there are proteins in the hemolymph which recognize microorganisms. When they recognize a microbe these will active proteins (receptors) that are in the cell membrane of liver cells. This protein in turn will initiate a cascade of several different proteins which will eventually send a transcription factor (a protein that induces the production of other proteins trough DNA and RNA) into the cell nucleus and initiate the production of RNA. This RNA will be secreted into the cytoplasm and here it will be translated into protein (also see DNA, RNA and protein). These proteins (AMPs) are secreted into the hemolymph (or other tissue depending were the infection is) and attack and kill the microorganisms. I illustrated the process in a movie:

The local immune response

Insects can also synthesize AMPs in epithelia. Epithelia line the outside of the insect (skin) and the intestine of insects, so these are exposed first to microorganisms. Although there are differences with the systemic immune response the general idea is the same. After recognition of microorganisms by extracellular (outside the cells) proteins the chain reaction will be initiated and induced the production of AMPs. These will fight the infection.

So to conclude, insects have a potent immune defense against microorganisms. We can roughly divide the response into the cellular and the systemic immune response. These defenses help insects survive in a world full of potentially lethal microorganisms.

Source:

1.            Lemaitre B., Hoffmann J. 2007 The Host Defense of Drosophila melanogaster. Annual Review of Immunology 25(1), 697-743. (doi:doi:10.1146/annurev.immunol.25.022106.141615).

 

Insect anatomy scheme

A- Head
B- Thorax
C- Abdomen

  1. antenna
  2. ocelli (lower)
  3. ocelli (upper)
  4. compound eye
  5. brain (cerebral ganglia)
  6. prothorax
  7. dorsal artery
  8. tracheal tubes (trunk with spiracle)
  9. mesothorax
  10. metathorax
  11. first wing
  12. second wing
  13. mid-gut (stomach)
  14. heart
  15. ovary
  16. hind-gut (intestine, rectum & anus)
  17. anus
  18. vagina
  19. nerve chord (abdominal ganglia)
  20. Malpighian tubes
  21. pillow
  22. claws
  23. tarsus
  24. tibia
  25. femur
  26. trochanter
  27. fore-gut (crop, gizzard)
  28. thoracic ganglion
  29. coxa
  30. salivary gland
  31. subesophageal ganglion
  32. mouthparts

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