Until quite recently, it was generally believed that each individual is born with a fixed number of nerve cells in the brain, and that these cells gradually degenerate and die during the person's lifetime and cannot be replaced. This theory was disproved when researchers discovered that certain regions of the adult brain do in fact retain their ability to support and promote cell renewal (neurogenesis) throughout life, especially under conditions of mental stimuli and physical activity. One such brain region is the hippocampus, which subserves certain memory functions. But how the body delivers the message instructing the brain to step up its formation of new cells is yet unknown.
The central nervous system (CNS), comprising the brain and spinal cord, has been considered for a long time as "a forbidden city", in which the immune system is denied entry as its activity is perceived as a possible threat to the complex and dynamic nerve cell networks. Furthermore, immune cells that recognize the brain's own components("autoimmune" cells) are viewed as a real danger as they can induce autoimmune diseases. Thus, although autoimmune cells are often detected in the healthy individual, their presence there was perceived as an outcome of the body's failure to eliminate them. But Schwartz's group showed that these autoimmune cells have the potential ability ?if their levels are controlled ?to fight off debilitating degenerative conditions that can afflict the CNS, such
Source:American Committee for the Weizmann Institute of Science