In this first of two articles on protective barriers and immunity, we look at the physiological and anatomical barriers which protect the bee. The honey bee colony is composed, on average, of about 50 000 individuals living together in close proximity. Such crowded conditions, together with drifting and robbing, provide an ideal environment for the establishment and spread of microbial infections. Honey bees, like virtually all living organisms, are susceptible to a variety of pathogens, predators and pests that may have harmful effects on the development and productivity of the colony. This social insect has developed effective mechanisms that efficiently protect both the individual bee and the whole colony against a great number of microbial pathogens.