Methods and products for protecting against abscess formation associated with surgery, trauma or diseases that predispose the host to abscess formation are provided. Methods for forming immunomodulators and pharmaceutical compositions relating thereto also are provided. The products useful in the invention are polysaccharides including a repeat unit having a positively charged free amino group and a negatively charged group. The preferred polysaccharide is B. fragilis capsular polysaccharide A.