Think back to the last time you were sick. Did you feel like doing the Riverdance or retreating to a dark room to hibernate for hours?
When the body is sick, the brain responds with “sickness behavior,” a collection of symptoms that make you feel terrible but serve an important biological function. Fatigue, low mood, reclusiveness, pain and appetite loss are a few symptoms that go hand-in-hand with fever–a primary indicator of immune activation.
While most people attribute these symptoms to the infection, the immune system is actually to blame. When pathogens invade the body, immune cells send out substances called cytokines that tell the brain to react in this way. Mental symptoms like fatigue and reclusiveness conserve energy to help the body fight illness and prevent spreading it to others.
They also look a lot like clinical depression.
If the immune system can trigger mental symptoms during infection, could it also play a role in mental illnesses like depression? Many scientists think so.
Depression–both unipolar and bipolar–is common in patients with autoimmune disease (conditions in which the immune system attacks the body’s own tissues). What’s more, mental symptoms often wax and wane in the same pattern as flare-ups/remissions of the autoimmune disease itself.
But mental symptoms are not limited to autoimmune conditions. Any illness that causes chronic inflammation can activate these nasty mental symptoms. This includes allergies, chronic bacterial or viral infection, heart disease and many others. Interferon–a cytokine-producing drug used to treat hepatitis–causes serious depression in up to 40% of users. Other common side effects include mania, fatigue, cognitive dysfunction and anxiety.
If inflammation is truly linked to mental illness, you’d expect symptoms to improve when taking anti-inflammatory drugs. And in many cases, they do. Patients most likely to respond to anti-inflammatory drugs are those with severe, treatment-resistant depression.
While more research is needed, one thing is certain: the brain/immune connection is strong. No amount of psychoanalysis or Prozac is going to change that.