Certain zwitterionic polysaccharides, including those naturally expressed by commensal B. fragilis in the gut, interact with cells of the immune system and affect the TH1/TH2 balance so as to promote health. Nutritional formulas and nutritional supplements containing isolated preparations of such zwitterionic polysaccharides, and methods for preparing the nutritional formulas and supplements, are provided. Also provided is a method of promoting immune system maturation in an infant involving enteral administration of a nutritional formula or nutritional supplement of the invention.
