A substance found in many foods, including turkey, can suppress an overactive immune system, researchers are reporting. The substance, tryptophan, produces a breakdown product in the body that, in the study, reversed paralysis in mice with an experimental form of multiple sclerosis, an autoimmune disorder that attacks the fatty cells that insulate neurons. "I have always been a skeptic regarding the interaction of diet and immunity," said Dr. Lawrence Steinman, the chairman of the immunology program at Stanford, who led the study, published in Science last week. "But now I'm getting smacked in the head by my own research." Dr....