The present invention provides a component having effects in vivo of improving or optimizing the intestinal environment, suppressing intestinal putrefaction, or suppressing alternation of intestinal bacterial growth and/or pathological changes of intestinal bacterial growth in gut microbiota. The invention also provides an active ingredient suitably used for treating intestinal diseases. The invention provides a monoclonal IgA antibody that binds to amino acids 11 to 333 of serine hydroxymethyltransferase.