Immunity in Insects

Insects represent the largest class of animals. They solely rely on innate immunity to counter infection by the many microorganisms to which they are confronted. Experiments conducted on the model organism Drosophila melanogaster have provided genetic evidence that insects rely on both cellular and humoral mechanisms to control infections. Once epithelial barriers have been breached, different subsets of differentiated blood cells or hemocytes neutralize bacteria, fungi, or parasites by phagocytosis, production of microbicidal compounds, or encapsulation. In addition, the fat body secretes high amounts of antimicrobial peptides active on fungi and bacteria in response to infection. This induced response involves activation of the evolutionarily conserved Toll and immune deficiency signaling pathways, which promote nuclear translocation of transcription factors of the NF-κB family. While some aspects of insect immunity are ancient and have been conserved through evolution, some differences can be noted with the vertebrate innate immune system. These include the families of pattern recognition receptors used and the reliance on RNA interference as a major antiviral pathway.