The concept of autoimmunity was first predicted by Nobel Laureate Paul Ehrlich at the start of the twentieth century, and he described it as 'horror autotoxicus'. His experiments led him to conclude that the immune system is normally focused on responding to foreign materials and has an inbuilt tendency to avoid attacking self tissues. But when this process goes wrong, the immune system can attack self tissues resulting in autoimmune disease. The perplexing issue of what allows the immune system to attack self tissues is a continuing focus of research, as the following Collection of reviews demonstrates.