Depression is a condition, which includes a combination of symptoms that interfere with the person’s daily activities. Criteria for a major depressive disorder may include depressed mood most of the day or nearly every day, diminished interest or pleasure, significantly poor or increased appetite, weight loss/weight gain, insomnia or hypersomnia, irritability, agitation, lack of concentration, fatigue or loss of energy nearly every day, and recurrent thoughts of death.

Depression is a physical and mental disease caused by a chemical imbalance and dysregulation of neurotransmitter activity. Depression has been described as a disorder of immune suppression and immune activation. In depression, there is biological evidence of impairment of the cellular immunity and inflammation. Many studies have associated depression with significant risk for coronary heart disease, cancer, and diabetes, as well as higher mortality risk. Decreased natural killer cell cytotoxicity, which signifies impaired cellular immunity and inflammation has been associated with depression. The presence of these immunological markers has been identified in other medical illnesses, suggesting that immune impairment may be a common feature in both depression and in other medical comorbidities.

Continuous overproduction and increased circulation of cortisol in the organism of depressed patients have multiple implications for medical morbidity. Cortisol suppresses the cellular immune response necessary in protection of the organism against pathogens and viruses. Increased cortisol levels are also known to damage lymphoid organs and peripheral tissues.

There is no single cause of depression, but several factors play a role in the onset of the disorder. These factors include genetic predisposition, psychological vulnerability, biological factors, acute and chronic stressors, brain trauma. Each individual has a certain number of risk factors for the development of depression. The more risk factors the individual has, the greater the chance of the onset of a depressive disorder.

Genetic predisposition has a strong association with the development of the disorder, however, in unlikely to cause depression alone. Other factors in the environment usually trigger the development of the disorder. These factors may be related to traumatic childhood experiences or chronically stressful adult life.

Psychological vulnerability refers to personality style and the coping strategies used by the individual to deal with problems. Individuals with low self-esteem, low self-efficacy, utilizing avoidance and denial as coping strategies may be at a greater risk for depression. Acute and chronic stressors in the form of childhood trauma, losses of loved ones, divorce, family conflicts, unemployment, financial problems, living in abusive and violent relationships can trigger the onset of depression.

Biological factors in the form of hormonal and chemical imbalances in the brain have been recognized as depression triggers for quite some time. Several behavioral factors can act as attributing factors to the immune system’s weakening in depression. Such factors include substance abuse, dysfunction of healthy sleep cycle, impairment of healthy behavior pattern (including lack of physical activity/exercise, poor nutrition). Depressed individuals are suffering from the impairments of the sleep cycle, insomnia, further reinforced by the lack of physical exercise and alteration of eating patterns.

Overall, depression leads to a reduction of health behaviors. Personality difference and individual coping abilities play a major role in stress response. Some people may become depressed with little or no stress in their lives, while other people may resist high levels of stress with no damage to their health. Therefore, it is important to carefully assess the personality traits and genetic predispositions when diagnosing depression.