A T cell receptor that recognizes a human protein fragment (left) is remarkably similar to one that recognizes a bacterial protein fragment (right), and to two receptors capable of recognizing both human and bacterial protein fragments (middle). A study by researchers at Washington University School of Medicine in St. Louis and colleagues at Stanford University and Oxford University supports the idea that some T cells that react to microbes also may react to normal human proteins, causing autoimmune disease. The findings promise to accelerate efforts to improve diagnostic tools and treatments for autoimmune diseases.