Biochemistry of Insect Immune System

The multicellular organisms has been encountered by a diverse array of pathogens. In response to the foreign invaders, the insects has been reported to develop an immune system which is basically the interaction between the virulence of the pathogen and the defending capacity of the host insects. The immunity system in insects may be divided into basically innate and adaptive type immunity, but in insects only innate immunity is functional and the adaptive immunity is being absent in insects unlike mammals. Furthermore, the innate immunity is divided into cellular and humoral immunity in insects. Cellular immunity is being imparted by various haemocytes such as, plasmatocytes, granulocytes and oenocytoids and the humoral immunity is provided by various Anti Microbial Peptides (AMPs) which are produced by fatbodies. Behavioural immunity includes the avoidance and antiseptic behavior by the host insects towards the pathogens or the products of pathogens. The insect has to overcome a series of barriers before reaching the haemocoel. The cuticle, trachea and midgut act as major site for invasion by the nonself microbes. The cuticle or integument is the outermost layer for taget. It is chemically composed of chitin which are crosslinked with various types of proteins. Integument is the primary target for fungi on which the fungal spore adhere and germinate. After overcoming the morphological external barriers in insects the pathogen has to gain access to haemocel by overcoming the physiological immunity in insects. Physiological immunity basically comprises of the cellular immunity and the humoral immunity. For the activation of physiological immunity in insects, the identification of nonself is the most important. Recognition of nonself occurs by the help of fat body cells, hemocytes, midgut epithelium and cuticular epithelium. Thus different biochemical pathways such as IMD, TOLL, JAK-STAT are activated in response to the nonself invaders. The study if immunity in insects can help in better understanding and effective utilization of entomopathogens for the control of insects.