A type of beneficial bacterium takes hold in the gut by using a protein produced by the host's immune system, a new study in mice reports. The findings present a new function for this protein, called immunoglobulin A (IgA), the effects of which are typically studied in the context of infection, where it helps to clear pathogens. Bacteroides fragilis is an important member of the human gut microbiome, with evidence suggesting that it helps alleviate inflammatory symptoms in animal models. A recent study found that B. fragilis excreted in feces is bound to IgA. Here, Gregory Donaldson et al. sought to better explore the relationship. Previously, the researchers had discovered a gene in B. fragilis, ccf, that's essential for these bacteria to take hold in the gut. In a series of experiments, they assessed how this gene helps bacteria colonize the mucus lining of the gut. Among other findings, the researchers report that in mice unable to produce IgA, microscopy reveals that B. fragilis did not aggregate along the intestinal lining. The authors cite other research that shows that IgA-coated bacteria from healthy humans protects mice from disease - while IgA-coated bacteria from individuals with inflammatory bowel disease or nutritional deficiencies exacerbate these pathologies in mice. Therefore, the authors propose that during circumstances free from disease, IgA fosters colonization of microbiota with beneficial properties, while disease states may induce (or be caused by) IgA responses to pathogens that disrupt a healthy microbiome.