The capsular polysaccharide complex (CPC) of Bacteroides fragilis exhibits unusual biological properties. This polysaccharide complex promotes the formation of intraabdominal abscesses and, when administered systemically, can prevent abscess induction in a rat model of intraabdominal sepsis. Each of these biological properties is mediated by a T cell-dependent immune mechanism. The CPC consists of two distinct polysaccharides, PS A and PS B, each with repeating units that have positively charged amino groups and negatively charge carboxyl or phosphate groups. Analysis of these polysaccharides as well as of other charged polysaccharides of bacterial origin, before and after chemical modification, revealed that the oppositely charged groups are required for promotion of intraabdominal abscesses as well as for the distinct properties associated with the B. fragilis CPC and delineate one mechanism by which these biological responses occur.