Calcium signal in cell nucleus regulates not only many brain functions but also defence reactions of the immune system. Cells of the immune system can distinguish between protein molecules that are "self" and "non-self". For example, if we are exposed to pathogens such as bacteria or viruses that carry foreign molecules on their surface, the body reacts with an immune response. In contrast, cells are "tolerant" of the body's own molecules. This state of unresponsiveness, or anergy, is regulated by a cellular signal, a calcium-controlled switch that was known to control also many brain functions.