Immunity-Related Causes

People with depression tend to have higher overall inflammation or over-activated immune systems.

Because depression is so common, some scientists have theorized that it may be an evolutionary byproduct of our immune systems, that depression might literally be hard-wired into our brains (Miller, A., Timmie, W., 2012). Several researchers have found that people with depression tend to have higher levels of immune system inflammation or over-activated immune systems, even when they’re not fighting infections from other illnesses. And so many of the genetic variants that have been linked to depression have also been found to affect immune system function. Could this have caused depression to stay embedded in the genome?

Perhaps, if you view conditions like fever, fatigue, social avoidance and anorexia as adaptive behaviors to contain infection. Perhaps this helps explain why stress is such a strong risk factor for depression: the link between stress and depression might be the byproduct of a process that preactivates the immune system in anticipation of injury. Similarly, a disruption of sleep patterns is common to both mood disorders and immune system activation.

Stress is a strong risk factor for depression.

We know that severe, chronic stress depresses immune function, and immune dysfunction diminishes the body’s ability to fight any disease or disorder. Every cell contains a tiny clock called a telomere, which shortens each time the cell divides. Telomeres are protective caps on the ends of chromosomes that serve as indicators of aging because they naturally shorten over time. But they’re also highly susceptible to stress and depression (Wikgren, et al, 2012). Short telomeres and premature telomere shortening have been linked to depression, aging, HIV, osteoporosis, heart disease and other health conditions.

Cortisol, The Stress Hormone

The stress hormone cortisol suppresses immune cells’ ability to activate telomerase, an enzyme within each cell that keeps immune cells young by preserving their telomere length — that is, their ability to keep dividing and generating new cells (Effros, 2008). Under stress, the body boosts production of cortisol to support our “fight or flight” response. Elevated cortisone levels, however, wear down the immune system. Short-term stress actually “revs up” the immune system — an adaptive response that prepares our bodies for illness, injury or infection. But long-term or chronic stress causes too much wear and tear, and the system breaks down.

Chronic stress is taxing to the body.

Chronic or long-term activation of the body’s stress-immune response system leads to allostatic load, which refers to the physiological consequences, the prolonged wear-and-tear on the body of chronic stress. (Think of the impact of allostatic load on the human body as being similar to years of wear-and-tear on vehicles or appliances.) Allostatic load is measured as a composite index of indicators of cumulative strain on various organs and tissues, especially the cardiovascular system.

Allostatic load, the physical consequences of chronic stress, is often high in people with depression and anxiety and is associated with a variety of medical conditions.

Allostatic load levels are often high in depressed or anxious people. Allostatic load is also associated with impaired immunity, accelerated atherosclerosis and increased incidence of type 2 diabetes, obesity, hypertension (high blood pressure), hyperlipidemia (excess of fats in the blood). Allostatic load is also associated with osteoporosis (bone demineralization), due to its chronically high levels of havoc-wreaking cortisol and atrophy of nerve cells in the brain.

This stress load includes inflammatory chemicals such as cytokines, which have a significant impact on behavior and emotion.

Cytokines are regulatory proteins released by immune system cells that acts as mediators between cells when the body is generating an immune response. Cytokines can both produce symptoms of depression, and depress the immune system.

Depression frequently occurs in those with immune disorders. Activation of the immune system induces “sickness behavior” such as apathy, lethargy, lack of motivation and appetite dysregulation — all of which, you’ll note, just happen to be symptoms of depression.