evidence: The rapid increase in the prevalence of autoimmune disease over the last century cannot be simply attributed to genetic changes, but suggests either the removal of protective factors or the introduction of susceptibility factors in the environment1. Several environmental risk factors, such as low vitamin D, infection with Epstein-Barr virus, and smoking, have been associated with the occurrence of autoimmune disease2,3. However, compelling epidemiological evidence indicates an inverse correlation between infections with helminth parasites and the incidence of autoimmune disease worldwide4,5,6,7. These observations concur with the so-called ‘old friends’ hypothesis, suggesting that the long evolutionary co-adaptation between parasites and humans has fundamentally affected the composition of the human immune response. Therefore, by removing the regulatory influences of parasites from human populations attuned to co-exist with them, an imbalance in the immune system is established, thereby increasing susceptibility to immune-mediated disease8. Accordingly, helminth parasites likely constitute beneficial commensals rather than harmful pathogens9,10,11.