The most basic innate response to bacterial or fungal infection is a cellular response (Jiravanichpaisal et al., 2006) that immobilizes the invading microbe by phagocytosis, engulfment or a Host-pathogen interactions in Drosophila PERSPECTIVE melanization reaction that traps it (Fig. 1B) . Pathogens can also be immobilized in flies and other insects by a clotting reaction (Dushay, 2009) . Once immobilized, the pathogen can then be either destroyed extracellularly by antimicrobial peptides (AMPs) or eliminated intracellularly. Three basic types of Drosophila blood cell (known as hemocytes) perform these functions: plasmatocytes, which are professional phagocytic cells akin to mammalian macrophages; lamellocytes, which wrap themselves around invading microorganisms to form an enveloping capsule; and crystal cells, which contain the enzymes that catalyze melanization (Meister, 2004) (Fig. 1B) . As discussed later in more detail, many host genes that are required for phagocytosis have been identified using Drosophila in a series of genome-scale cell-based screens. Similar studies in the future might shed light on genes that are essential for lamellocyte and crystal cell function. Autophagy is another general mechanism important for clearing bacteria (Yano et al., 2008) and viruses (Cherry, 2009; Shelly et al., 2009) . It should be pointed out, however, that autophagy can also be hijacked for the benefit of the pathogen, as in the case of poliovirus, which derives its envelope membranes from autophagic vesicles (Suhy et al., 2000) .