BIOLOGICAL MEDIATORS OF INSECT IMMUNITY

Infection in insects stimulates a complex defensive response. Recognit ion of pathogens may be accomplished by plasma or hemocyte proteins th at bind specifically to bacterial or fungal polysaccharides. Several m orphologically distinct hemocyte cell types cooperate in the immune re sponse. Hemocytes attach to invading organisms and then isolate them b y phagocytosis, by trapping them in hemocyte aggregates called nodules , or by forming an organized multicellular capsule around large parasi tes. These responses are often accompanied by proteolytic activation o f the phenoloxidase zymogen that is present in the hemolymph. A compon ent of insect immune responses to bacteria is the synthesis by fat bod y and hemocytes of a variety of antibacterial proteins and peptides, w hich are secreted into the hemolymph. These molecules attack bacteria by several mechanisms. Inducible antifungal proteins have also been re cently discovered in insect hemolymph. The promoters for several antib acterial protein genes in insects are regulated by transcription facto rs similar to those involved in mammalian acute phase responses.