 The secretory IgA, SIGA, antibody class plays a key role in protecting the body from infections by blocking access of pathogens and their secreted products into the body proper. It also helped shape the composition of the gut microbiota by binding to gut bacteria and transporting them into the outer layer of mucus, thereby creating a barrier between the bacteria and the inner mucus layer. This interaction is mediated by the polymeric immunoglobulin receptor, PIGR, on the epithelial cell surface, which binds to SIGA and transports it into the outer mucus layer. Without this interaction, SIGA would be unable to anchor itself in the outer mucus layer, allowing pathogens to enter the inner mucus layer and potentially cause infection. This article was authored by Eric W. Rogier, Aubrey L. France, Maria E. C. Bruno, and others.