In the developing human fetus, cell suicide also plays a major role in the formation of the nervous system. At a certain stage of fetal development, these neurons begin generating enormous numbers of nerve fibers, which they simply cast out in the general direction of tissues and cells needing nerve connections. From the moment a neuron is spun out of the central nervous system toward potential target cells, it is destined to die. In some respects this seems an incredibly wasteful way to build a nervous system. The same thing happens with the cells of our immune system.
Death is the default state. .
Cells of the immune system, including the warrior cells we met earlier, are also generated in great excess. As when neurons make contact with a targeted cell, lymphocytes detecting a foreign protein or engaging a virlly infected cell are rewarded with growth factors that switch off their death program. The resulting cell may survive five years, or ten, or even for the life span of its host, providing a type of 'memory' of pathogens previously encountered by the immune system. But if they fail to find and eliminate a foreign invader during the allotted trial period, they are invited, in effect, to fall on their swords. Again, death is the default state for these white blood cells.