ABSTRACT
Immunity describes the ability of organisms to resist infections and diseases. The immune system encompasses all structures and molecules contributing to the recognition of pathogens and defense against them. Insects are the most successful group of organisms in terms of biodiversity, and among many other factors their powerful immune system has contributed substantially to this ecological dominance. Insects are protected by staggered defense mechanisms that provide immunity against microbial pathogens. Their sclerotized chitinous integument forms an effi cient primary barrier against most microorganisms, and only parasitic fungi can directly infect insects by breaching the exoskeleton because they secrete enzymes that degrade proteins and chitin to facilitate penetration (Gillespie et al. 2000). Insects lack an antibody-based adaptive immune system as found in vertebrates, so antimicrobial defense relies on an innate immune system encompassing cellular and humoral components. The cellular defense mechanisms include phagocytosis and the multicellular encapsulation of microbes and parasites that have gained access to the hemocoel. Immunity-related cells circulating in the hemolymph can distinguish between host and non-host surfaces, adhering specifi cally to pathogen-associated molecular patterns that are not found on insect cells. For example, lipopolysaccharides or peptidoglycans in bacterial cell walls, or β-1,3 glucans representing the major components of fungal cells, are recognized by insect receptors that in turn trigger further immune responses.
