Polysaccharide A (PSA) produced by the human symbiont Bacteroides fragilis can induce Foxp3+ Tregs to produce IL-10 and TGF-β2 through Toll-like receptor 2 (TLR2) expression, thereby regulating inflammation and alleviating the progression of pulmonary fibrosis. The gene discussed is TLR2; the disease is pulmonary fibrosis.