Immune system activation affects our brain, and alters our behavior. It can cause:
- Fever
- Loss of appetite
- Lethargy/fatigue
- Anhedonia (reduced interest in things we found pleasurable)
- Reduced social behavior
- Hyperalgesia (sensitivity to pain)
- Poor memory/concentration
These non-specific symptoms that accompany immune response are called “sickness behavior.” It occurs because the cytokines (IL-1 and IL-6) produced by macrophages when our non-specific immune system is activated communicate the brain, “I am sick.”
According to Hart (1988), these behavior changes are adaptive, and designed to help promote recovery. They are not a sign of weakness. Rather they are just the body doing what it is supposed to do in order to fight off disease. For example:
- The cortisol and catekolines produce energy (needed to produce fever), which slows down bacteria replication.
- Reduced activity conserves energy, which is needed to fight disease.
People who are depressed look like they are stressed. In fact, depression is very similar to sickness behavior according to Maes’ macrophage theory of depression (1993/1995) . Major depression activates immune system cells and cytokines in the blood. Cases of depression that are resistant to treatment have been shown to have higher cytokines. When people are administered cytokines to boost their immune system, they get depressed.