Life-history theory suggests that breeding effort leads to lowered subsequent fecundity and survival. Immunity is a major physiological mechanism regulating survival and trade-offs between reproductive effort and immune function may be expected. In cooperatively breeding species, breeders may benefit from non-breeders that perform energetically costly tasks by increased survival. However, this has never been related to immune function. In mammals trade-offs between reproductive effort and immune function are likely to be mediated through energy that can be stored in adipose tissue. We compared fat mass and immune function measured as spleen mass between breeders and non-breeders and among the sexes in cooperatively breeding Natal mole-rats Cryptomys hottentotus natalensis. Assuming that larger spleen size indicates a stronger immune function, we hypothesized that reproductive effort leads to reduced energy stores and thus smaller spleen mass in breeders and expected females to have a larger fat and spleen mass than males. We could establish a relationship between energy stores and spleen mass but found the expected sex-specific difference only in breeders. Spleen mass was similar between breeders and non-breeders. The energy cost of reproduction may be compensated for by the contribution of non-breeders in this cooperative breeder. We suggest that non-breeders reduce work loads for breeders and thereby allow breeders a higher investment in immune function that may result in improved survival. This could also explain increased survival in mole-rats and possibly other cooperative vertebrates.