Abstract

It is well known that organisms defend their fitness against attack from parasites and pathogens by mounting a personal immune response. However, there is increasing evidence that organisms from diverse taxa also exhibit immune responses for the purpose of protecting other individuals as well as themselves. We argue that any type of immunity that has fitness consequences for both the challenged individual and one or more recipients should be referred to as ‘social immunity’. We show that social immune systems are a widespread yet relatively neglected component of immunity, ideal for the study of social evolution. Whereas personal immune systems protect lifespan, social immune systems effectively defend the fecundity component of fitness, commonly protecting offspring or reproductive kin. We suggest that there are likely to be close links between life history and the extent of investment in each form of immunity. Furthermore, trade-offs between social and personal immunity may explain individual variation in personal immune responses, including sex-specific immune defences.