All insects defend themselves against bacteria and parasites, using cellular and humoral systems that are rapidly activated in infected animals. Among the induced effector molecules are antibacterial proteins and peptides such as cecropins, attacins, lysozymes and insect defensins, the genes for many of which have now been cloned. The induction of this system, which lacks clonally selected receptors like those of vertebrate B and T cells, may provide a model for innate immune reactions in other animals, including vertebrates.