These tiny cells are scattered throughout the brain, lying dormant waiting to be activated. In fact, they are activated by many stimuli and are quite easy to activate. For our discussion, activation of the body's immune system by vaccination is a most important stimuli for activation of brain microglia. Numerous studies have shown that when the body's immune system is activated, the brain's immune cells are likewise activated. This occurs by several pathways, not important to this discussion. The more powerfully the body's immune system is stimulated the more intense is the brain's reaction. Prolonged activation of the body's immune system likewise produces prolonged activation of the brain's immune system.