The concept of innate immunity refers to the first-line host defense that serves to Limit infection in the early hours after exposure to microorganisms, Recent data have highlighted similarities between pathogen recognition, signaling pathways, and effector mechanisms of innate immunity in Drosophila and mammals, pointing to a common ancestry of these defenses. In addition to its role in the early phase of defense, innate immunity;in mammals appears to play a key role in stimulating the subsequent, clonal response of adaptive immunity.