Understanding the Relationship between Depression and the Immune System

The immune system is one of the body's most valuable assets. Immune cells race to the site of a new infection or injury, where they work tirelessly against bacteria and viruses or repair tissue damage.

Yet depression and the immune system are inextricably linked. Research shows, for example, that depression is more common in people with other disorders of the immune system, such as diabetes and rheumatoid arthritis. For instance, diabetes doubles the risk of depression . At the same time, people with depression have up to t hree times as many inflammatory cells as those without depression. This excess of inflammatory cells signals that the immune system isn't functioning normally. People with treatment-resistant depression are even more likely to have elevated inflammatory markers.

However, these links also mean that seeking depression treatment can benefit your overall health. Read more for a closer look at the relationship between depression and the immune system.

While evidence shows that depression and the immune system are connected, researchers don't fully understand that connection, though they have established some theories.

For example, chronic stress—an integral feature of depression—raises levels of a stress hormone called cortisol. This affects microglia cells, traditionally known as "housekeeper" cells because they calmly protect healthy nerve cells and efficiently remove debris. Their ability to migrate quickly to an accident site (any part of our body that is infected or injured) to do so highlights their crucial role in the body's immune response.

A stress-related cortisol surge causes previously calm housekeeper cells to go "rogue." The microglia begin to churn out cytokines , which are pro-inflammatory proteins. These cytokines damage the microglia themselves, causing them to churn out even more cytokines. This inflammatory cascade damages synapses—the gaps between nerve cells—and provokes inflammation throughout the brain. The brain regions responsible for controlling mood and emotion are especially susceptible to this inflammatory attack.

These dramatic events decrease the availability of chemical messengers like serotonin and dopamine, thus interrupting normal communication between our brain cells. These chemical messengers are especially important in regulating sleep, appetite, emotion, and memory—all things that are disrupted in depression.

In short, chronic stress can contribute to impaired function of the immune system, and this triggers an inflammatory response in the brain, leading to the symptoms of depression.

It's important to understand the basics of how depression and the immune system interact, but specific examples can help illuminate the relationship.

You're more likely to develop a common cold , for instance, if you've experienced ongoing psychological stress. Not only that, but a cold can linger for longer, far outstaying its welcome. Scientists believe this is because the immune system is dialed up with chronic stress, leading to excess inflammation.

If you have depression, you could have a greater tendency toward cardiovascular disease , another disorder related to inflammation. Up to 40 percent of people with heart disease develop depression at some point in their lives, and those with heart disease develop depression at three times the rate of the general population. People with depression have a 64 percent greater risk of developing coronary artery disease .

Evidence also points to a higher risk of cancer in people with depression. This could be related to a greater tendency of some patients with untreated depression to smoke, drink alcohol to excess, or remain sedentary—all risk factors for cancer. But there also appears to be a consistent, independent link between depression and an impaired immune system in cancer: the same pro-inflammatory proteins, cytokines, exist in both conditions. This association has been reported in pancreatic cancer and ovarian cancer , among others.

Reassuringly, treating depression could make a difference. Research published in the Journal of Clinical Oncology found that decreasing the symptoms of depression in women with advanced breast cancer was associated with longer subsequent survival.

Indeed, seeking treatment for depression can help your physical and mental health alike. One study followed people over eight years and found that treating depression as soon as possible decreased their risk of strokes and heart attacks by half. Depression treatment has also been found to improve sugar levels , decrease pain , and boost cardiovascular health.

Remarkably, depression treatment can cause inflammation in the brain to vanish, showing how closely depression and the immune system are intertwined. A major study in the Lancet journal focused on people with longstanding untreated depression. These patients had high levels of immune system activation (as measured by activated microglia cells) that built year on year in brain regions crucial for mood and behavior. However, when these patients received antidepressant treatment, this yearly increase in inflammation disappeared.

Given the far-reaching impact of depression treatment on the brain and body, what can you do if depression medication hasn't provided the symptom relief you hoped for? One non-drug treatment option for depression is Transcranial Magnetic Stimulation (TMS). TMS applies highly focused magnetic pulses to the brain regions that regulate mood. TMS has been cleared by the FDA for Major Depressive Disorder and has been shown to decrease signals of brain inflammation in people with depression.

Whichever treatment option you choose, one thing is clear: proactively addressing your symptoms of depression can enhance your overall health. Dialing down your immune system to a healthy level by reaching out for depression treatment is a way to heal your body and mind.