The infant gut microbiota plays a vital role in immune development. Secretory IgA (SIgA), the most abundant antibody in the gut, is a key modulator of microbiota composition throughout life. Maternal milk is the primary source of intestinal SIgA for the developing infant, but the ability of maternal milk SIgA to guide microbe-mediated immune imprinting remains poorly understood. Here, we characterize the microbiota reactivity of human milk SIgA, demonstrating relationships between maternal SIgA and infant gut microbiota composition. This leads to the identification and characterization of E. ramosum, a microbe that is controlled by milk SIgA and plays an important role in immune development worthy of further consideration in studies of allergy prevention at the maternal–infant interface.
