if and in treatment window otherwise 0 ⎩ ⎩ Immune pathways and defence mechanisms in honey bees Apis mellifera Social insects are able to mount both group-level and individual defences against pathogens. Here we focus on individual defences, by presenting a genome-wide analysis of immunity in a social insect, the honey bee Apis mellifera. We present honey bee models for each of four signalling pathways associated with immunity, identifying plausible orthologues for nearly all predicted pathway members. When compared to the sequenced Drosophila and Anopheles genomes, honey bees possess roughly one-third as many genes in 17 gene families implicated in insect immunity. We suggest that an implied reduction in immune flexibility in bees reflects either the strength of social barriers to disease, or a tendency for bees to be attacked by a limited set of highly coevolved pathogens. While evident in social organisms ranging from humans to birds (Brown & Brown, 2004; Masuda et al ., 2004) , the impacts of sociality on disease are especially vivid within social insect colonies. Here, typically thousands of individuals interact in close quarters, at densities far exceeding those of even the most crowded vertebrate social groups (Wilson, 1971) . This density, coupled with a relatively homeostatic nest environment and the presence of stored resources, makes social insects attractive targets for disease agents (Schmid-Hempel, 1998) . As expected based on their parasite and pathogen pressures, social insects have evolved both individual and group strategies to combat disease. Grooming, nest hygiene and other behavioural traits found throughout the social insects can reduce the impacts of pathogenic bacteria, fungi and parasitic mites. For example, 'hygienic behaviour' first described for honey bees (Rothenbuhler, 1964) is now a classical example of a social defence, whereby workers identify and remove infected larvae from among the healthy brood (Spivak & Reuter, 2001) . Other defences enabled by sociality include the construction of nests from antimicrobial materials (Christe et al ., 2003) , the raising of offspring in sterile nurseries (Burgett, 1997) , social 'fever' in response to disease (Starks et al ., 2000) , transference of immune traits (Traniello et al ., 2002; Sadd et al ., 2005) , and heightened risk-taking by infected individuals (Schmid-Hempel, 2005) . Like most eukaryotes, colony members also possess individual defences, including immune responses toward disease agents (Casteels-Josson et al ., 1994; Evans, 2004) . The recent sequencing of the honey bee genome (Honey Bee Genome Sequencing Consortium, 2006) allows the first global analysis of immune components in honey bees, and the second opportunity (after humans) to use genomic insights to better understand disease resistance in a highly social organism.