Feeling anxious before a shot can make vaccinations significantly more potent:
From an evolutionary perspective, the fact that short-term stress revs up the immune system makes sense. Consider a gazelle fleeing a lioness. Once the gazelle’s eyes and ears alert its brain to the threat, certain brain regions immediately activate the famous fight-or-flight response, sending electrical signals along the nervous system to the muscles and many other organs, including the endocrine glands—the body’s hormone factories. Levels of cortisol, epinephrine, adrenaline and noradrenaline rapidly increase; the heart beats faster; and enzymes race to convert glucose and fatty acids into energy for cells. All these swift biological changes give the gazelle the best chance of escape. At the same time, [Stanford University's Firdaus] Dhabhar and others’ research suggests, the brain’s recognition of a threat prompts the immune system to prepare for potential injury. The spleen and other organs release immune cells specialized for identifying and destroying invaders and healing damaged tissues. After all, even if the gazelle escapes with its life, it may need to heal wounds sustained during its flight and prevent them from becoming infected.