Immune cells must learn not to attack the body itself. A team of researchers has discovered a previously unknown mechanism behind this: other immune cells, the B cells, contribute to the 'training' of the T cells in the thymus gland. If this process fails, autoimmune diseases can develop. The study confirms this for Neuromyelitis optica, a disease similar to Multiple Sclerosis. Other autoimmune diseases may be linked to the failure of this new mechanism as well.