Maternal antiviral history synergizes with pregnancy and lactation to transfer intergenerational systemic immunity through IgG in milk

Maternal immune transfer is essential for early-life health, yet whether immune experiences before pregnancy shape maternal physiology to optimize immunity in subsequent offspring is unclear. Here, we show that respiratory viral infection before pregnancy confers robust protection against lethal neonatal influenza through antibodies transferred postpartum in milk. Despite the predominance of IgA in milk, antiviral IgG is indispensable for protection. Pregnancy amplifies pre-existing antiviral B cell responses, while prior infection durably reprograms the mammary gland to promote transfer of circulating antiviral IgG into milk. These antibodies retain their epitope specificity, are enriched for broadly protective influenza epitopes, remain functional after passage through the neonatal intestine, and enter offspring circulation through FcRn to provide protection beyond weaning. Natural transmission of virus from infected offspring to mothers establishes maternal immunity that protects future offspring, revealing a coordinated adaptive program that links maternal immune history, pregnancy, and lactation to optimize intergenerational immunity.