The evolutionary strategy of transferring maternal antibodies via milk profoundly impacts the survival, lifelong health, and wellbeing of all neonates, including a pronounced impact on human breastfeeding success and infant development. While there has been increased recognition that interorgan connectivity influences the quality of a mother"s milk, potentially to personalize it for her offspring, the underlying bases for these processes are incompletely resolved. Here, we define an essential role of Peyer"s patches (PPs) for the generation of plasma cells that secrete maternal immunoglobulin A (IgA) into milk. Our metagenomic analysis reveals that the presence of certain residential microorganisms in the gastrointestinal (GI) tract, such as Bacteroides acidifaciens and Prevotella buccalis, is indispensable for the programming of maternal IgA synthesis prior to lactational transfer. Our data provide important insights into how the microbiome of the maternal GI environment, specifically through PPs, can be communicated to the next generation via milk.