Encapsulation and nodulation in insects

Evolution of the insect immune system led to the creation of a comprehensive cellular defensesystem, not only involving phagocytosis, but also encapsulation and nodulation (both often referred to as capsule formation) allowing the isolation and neutralization of invading pathogens and parasites. Such reactions are closely related to the anatomical and physiological characteristics in insects with their external skeleton and open circulatory blood system. Encapsulation and nodulation are most important defense mechanisms in insects, as they allow targeting of the immune response to the site of damage to quickly destroy the intruder. Host penetration results in both the production of damageassociated molecular patterns (DAMPs) and to the presence of pathogen-associated molecular patterns (PAMPs) in the hemolymph. Subsequent signal induction occurs by host pattern recognition receptors (PRRs) and other systems. Capsule formation results from aggregation and partialdisruption of the hemocytes on the target surface resulting in melanization by the proPO cascade. Reactive oxygen (ROS) and nitrogen (RNS) species are emitted during melanogenesis and targeted against the invader. As a result, the intruder is not only isolated within the capsule but also destroyed. Insects have a number of systems (serpins, antioxidants), aimed at the regulation of melanogenesis and inactivation of toxic products resulting from melanization. All these complex mechanisms allow rapid and effective detection, isolation and destruction of invaders with minimal damage to the insect.