Abstract
Cellular defences are accomplished by haemocyte-mediated responses such as phagocytosis and encapsulation. This review describes the current knowledge regarding cellular immune responses of insects of different orders at developmental stages of larvae, pupae and adults, emphasizing studies on different phylogenic groups of insects. Insect haemocytes originally evolved from mesodermally-derived stem cells that differentiated into specific cell lines, which are identified based on their forms, functions and molecular markers. In insects, most cellular defence responses involve granular cells and plasmatocytes, whereas in Drosophila they primarily involve plasmatocytes and lamellocytes. Insect haemocytes recognise a variety of foreign bodies as well as altered self components. Their cell-surface receptors are involved in these specific recognition events. Once a target has been identified as a foreign body, haemocyte-mediated defence responses are evoked by signaling factors and effector molecules that control cell adhesion and cytotoxicity. Several lines of evidence indicate that humoral and cellular defence responses are well coordinated with one another. Coordination between the immune system and the nervous system may also play a role in regulating inflammatory-like responses in insects during infection. The total haemocyte counts and the differential haemocyte counts vary in the different life stages. Foreign bodies and chemicals have also been reported to be factors that affect the number of haemocytes.