Summary: The authors analyzed gut microbiome data from infant stool samples and secretory immunoglobulin A (SIgA) antibody data from breastmilk samples collected through the Canadian Healthy Infant Longitudinal Development (CHILD) Cohort Study. They discovered that for one bacterium, Erysipelatoclostridium ramosum, higher levels of E. ramosum-binding SIgA in the milk corresponded to lower levels of the bacterium in the infant gut. Using human cell culture experiments, they found that E. ramosum activates the T helper 17 (Th17) immune response when it adheres to the intestine and that SIgA interferes with this mechanism. As the Th17 immune response is implicated in asthma, allergies, and other inflammatory diseases, this work suggests that E. ramosum and SIgA could serve as promising therapeutic targets for promoting healthy immune development in infants.