Knowing your friends: invertebrate innate immunity fosters beneficial bacterial symbioses

Invertebrates lack conventional antibody-based immunity but have the capacity to distinguish between symbionts and pathogens. In this Review, Nyholm and Graf describe how the innate immune system of four model invertebrates functions in the establishment and maintenance of bacterial symbioses. The innate immune system is present in all animals and is a crucial first line of defence against pathogens. However, animals also harbour large numbers of beneficial microorganisms that can be housed in the digestive tract, in specialized organs or on tissue surfaces. Although invertebrates lack conventional antibody-based immunity, they are capable of eliminating pathogens and, perhaps more importantly, discriminating them from other microorganisms. This Review examines the interactions between the innate immune systems of several model invertebrates and the symbionts of these organisms, and addresses the central question of how these long-lived and specific associations are established and maintained.