Associate Professor Dr. Masako Fujita co-authors article in Evolution, Medicine, and Public Health

Department of Anthropology Associate Professor Dr. Masako Fujita and co-authors Katherine Wander, Siobhan Mattison, Blandina Mmbaga and others publish in Evolution, Medicine, and Public Health.  

The article is titled “Tradeoffs in milk immunity affect infant infectious disease risk.” The article discusses research on milk immune activity, a new area of research, among almost 100 breastfeeding mother–infant dyads in Kilimanjaro, Tanzania.

The study used a new lab-based technique and described how mothers’ milk differed in their immune responses to some bacteria, and followed the infants to see whether those receiving milk with stronger responses in lab were less likely to develop infectious diseases.  

They found support for this with the infectious agent Salmonella. Infants receiving milk with stronger pro-inflammatory responses to Salmonella had lower risk for respiratory infections during the 2.5 months follow-up period. However, they also found the opposite pattern with a non-infectious bacterium. Infants receiving milk that responded strongly to the benign strains of E. coli (that tends to exist harmoniously in our digestive systems) had higher risk for gastrointestinal infections. Moreover, milk responses to Salmonella tended to co-occur with responses to E.coli, suggesting that milk with strong immune responses have potential to decrease risk for some infections but increase others among infants.  

Their findings make sense in that immune protections often come with collateral damage because the immune system does not always differentiate pathogenic from benign targets, giving rise to allergies and auto-immune conditions, for example. Still, the study’s discovery of protective and harmful effects of milk immune activity on infant infectious disease risk comes as a bit of surprise because we tend to think of milk as the mighty fluid that does no harm. The reality seems a bit more complicated, calling for future research to clarify how the immune system of milk has evolved to strike a balance between protection and harm.

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“Background and objectives

The human immune system has evolved to balance protection against infection with control of immune-mediated damage and tolerance of commensal microbes. Such tradeoffs between protection and harm almost certainly extend to the immune system of milk.


Among breastfeeding mother–infant dyads in Kilimanjaro, Tanzania, we characterized in vitro proinflammatory milk immune responses to Salmonella enterica (an infectious agent) and Escherichia coli (a benign target) as the increase in interleukin-6 after 24 h of incubation with each bacterium. We characterized incident infectious diseases among infants through passive monitoring. We used Cox proportional hazards models to describe associations between milk immune activity and infant infectious disease.


Among infants, risk for respiratory infections declined with increasing milk in vitro proinflammatory response to S. enterica (hazard ratio [HR]: 0.68; 95% confidence interval [CI]: 0.54, 0.86; P: 0.001), while risk for gastrointestinal infections increased with increasing milk in vitro proinflammatory response to E. coli (HR: 1.44; 95% CI: 1.05, 1.99; P: 0.022). Milk proinflammatory responses to S. enterica and E. coli were positively correlated (Spearman’s rho: 0.60; P: 0.000).

Conclusions and implications

These findings demonstrate a tradeoff in milk immune activity: the benefits of appropriate proinflammatory activity come at the hazard of misdirected proinflammatory activity. This tradeoff is likely to affect infant health in complex ways, depending on prevailing infectious disease conditions. How mother–infant dyads optimize proinflammatory milk immune activity should be a central question in future ecological–evolutionary studies of the immune system of milk.”