Masako Fujita

EMAILED ABSTRACT: The immune system of milk (ISOM) protects infants from infectious disease but must balance effective defense with the risk of immune-mediated harm. From an evolutionary perspective, ISOM should differentiate harmful pathogens from benign targets, though both protection and regulation are energetically costly and linked to maternal life-history tradeoffs. This study examined whether chronic maternal stressors—environmental, socioeconomic, nutritional, and psychosocial—affect ISOM competence, defined as the ability to respond appropriately to pathogens while avoiding unnecessary responses. Using in-vitro stimulation, milk interleukin-6 (IL-6) responses to Salmonella enterica (pathogenic) and Bifidobacterium breve (benign) were characterized among 77 mother-infant dyads in Michigan, USA. Responses were categorized as inert, non-specific (response to both), or appropriate and specific (response to Salmonella only). Multinomial logit models evaluated associations with maternal stressors. Poverty and overweight/obesity were associated with reduced probability of specific responses, suggesting disrupted ISOM competence. In contrast, autoimmune thyroid condition was positively associated with specific responses, indicating a possible tradeoff between immune investment and maternal autoimmune risk. Psychosocial stress showed no strong effects. Overall, ISOM competence appears to be buffered against psychosocial stress but sensitive to financial stress and nutritional imbalances, with implications for maternal life history energy allocation and infant immune development.