The Immune System
During acute stress, the immune system increases due to stress-induced trafficking. This is where cortisol stimulates immune system cells to leave the bloodstream and move to the lymph nodes and skin, thereby putting up a large defense.
Within an hour of this begging of stress, the opposite occurs. The HPA axis and the sympathetic nervous system begin to suppress the immune response and bring it back to its baseline or below it. Chronic stress can cause the immune system to fall 40-70% below the baseline due to suppressing the production of immune system cells and killing the body’s first line of defense against pathogens. Glucocorticoids (like cortisol) can trigger apoptosis (cell self-destruction) in immune system cells. One study showed that people are three times more likely to catch a cold if recently experiencing prolonged stress.
In addition to catching diseases from foreign molecules, the lowered immune system makes a person more likely for allergies and for autoimmune diseases, like type one diabetes or multiple sclerosis. Stress can make the conditions for autoimmune disease worse and evidence is showing to indicate that stress may play a role in the development of these diseases.