Clinical depression is a mental illness characterized by extreme anhedonia (inability to experience pleasure), apathy and fatigue.  It is grouped as a mood disorder amongst the mental illnesses, along with its cousin bipolar disorder. Its symptoms had been recognized for decades, but it was not until the 1980s that it was accepted that these symptoms were caused by the shortage of certain neural transmitters: serotonin, noreprinephrine, or dopamine. Depressed patients produce these neurotransmitters, but they cannot store and essentially "run out" of one or more on a regular basis. 

Antidepressants work by preventing neurons from "dumping" their supply, allowing them to slowly be built up in the neurons and released in a more or less normal manner. There are different classes of antidepressants, and each class works on each of the three different neurotransmitters depending on which neurotransmitter is the one that is being absorbed too quickly. It is also possible for more than neurotransmitter to be impacted, in which case the patient may have to be put on an antidepressant that works on two neurotransmitters or on more than one antidepressant.

Unfortunately, one of the risks when starting antidepressants is that the patient's symptoms other than depression will disappear, leaving the patient at a higher risk for suicide. Sometimes the antidepressants themselves can increase the risk of suicide because the patient can find the antidepressant's side effects unbearable. Doctors need to stay informed of the progress of any of their patients who are starting or switching antidepressants. 

Clinical depression is separate from the type of depression suffered by persons who have suffered a trauma, such as severe injury or the death of a loved one. This one is referred to as situational depression. However, it is possible for a person to have both situational depression and clinical depression at the same time or for situational depression to progress into clinical depression.

James Wilson suffers from clinical depression and is currently being treated with antidepressants. He believes House also suffers from clinical depression and gave him a dose of anti-depressants which had the effect of cheering House up.

Major depression at NIH

Major depressive disorder at Wikipedia

Depression at Mayo Clinic

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