Young mammals are unable to mount an efficient immune response against invading pathogens. Until their immune system is mature mothers transmit to their young immunological compounds during lactation. Given that genetic and foster mothers can assume this protective role, we propose that young mammals may gain immunological benefits by suckling more than one nursing female, a behaviour referred to as “allosuckling”. This hypothesis has so far not been considered as a potential explanation for the propensity of young mammals to suckle foster mothers. However, pathogen transmission through milk during allosuckling may reduce the immunological net benefit that young gain, and furthermore allosuckling may increase pathogen transmission between foster and genetic mothers implying costs of allosuckling for all participants. Here, we develop the immunological function of allosuckling hypothesis (IFA) as a potential explanation for intra-and interspecific variation in allosuckling frequency. We present published experimental evidence for the assumption that immunological benefits of allosuckling depend on the immunological status of the offspring, the foster and the genetic mothers. Finally, we give predictions arising from the IFA hypothesis and propose that the IFA may provide a new explanation as to why neonates suckle various females and why foster females often refuse to nurse nonoffspring.