Biology of Depression – Psychoneuroimmunology
The immune system is primarily composed of three different types of cells: T, B, and natural killer (NK) cells. Collectively, these cells are also known as lymphocytes or white blood cells. T cells seek out and destroy cells infected with pathogens (infectious agents such as bacteria, viruses, etc.). B cells produce antibodies which attack antigens (foreign molecules or organisms). T and B cells are both "antigen specific", meaning that they only spring into action when they have encountered a specific type of antigen in our bodies. However, NK cells do not require an antigen as a trigger for action. NK cells are continuously surveying our bodies to help keep us safe. When NK cells encounter an antigen, they kill it; thus, the name "natural killer" cell.
A strong immune system is necessary to ward off infections and to keep us healthy. Anything that compromises our immune system makes us vulnerable to illnesses. Importantly, it is not just biological pathogens that can compromise our immune system. Psychologically and socially stressful events such as the death of a loved one, severe abuse or trauma, marital separation, social failures, social isolation, or long-term caregiving can also weaken our immune systems.
There is much evidence available today to support the idea that psychological and social stressors can have a physical effect on the immune system. In laboratory research, animals subjected to loud sounds, intermittent shocks, or an inability to move around show suppressed immune cell functioning. In humans, psychological pathogens also have the same effect on the immune system; the more intense the stressors become, the more our defensive systems are weakened. Even stress caused by relatively minor aversive events such as academic examinations can cause temporary increases in white blood cell counts to occur.
Chronic stressors that last over periods of one or more years compromise immune function, lead to an increased risk of developing physical illnesses and also create an increased likelihood of becoming depressed. In rats, chronic stress causes the brain to secrete high levels of stress hormones, which then alter the neurotransmitter receptors for serotonin. In humans, chronic stress seems to influence the serotonin, norepinephrine, and dopamine neurotransmitter systems, particularly in individuals who are socially isolated and/or have poor coping skills.
A more recent line of research suggests that stress causes a decline in the rate of formation of new neurons (neurogenesis) in the part of the brain known as the hippocampus. Autopsy evidence suggests that depressed people who experienced chronic stress and then went on to commit suicide showed reduced neurogenesis in the hippocampus. In contrast, depressed people who benefit from taking antidepressants or receiving electroshock therapy both show increased neurogenesis.
The field of psychoneuroimmunology is relatively new. Future research will likely provide us with more clues about the interaction between the functioning of our immune systems and depression.