Forget what you think you know. It has nothing to do with serotonin or other neurotransmitters. OK, it has something to do with them, but that's not the key.
Evidence now indicates that all effective depression treatments turn on genes that produce nerve growth factor in brain cells. The nerve growth factor literally makes your brain grow; nerve cells sprout new branches, connections between neurons flourish, you learn and adapt more easily, regaining behavioral flexibility.
Antidepressants kick off a transient increase in neurotransmitter, which functions as an external messenger between one nerve cell and the next one down the line. The neurotransmitters important in depression are serotonin, norepinephrine and dopamine. They get taken up on the receiving cell by a receptor that sits on the fat-rich surface of the cell and is coupled to a protein inside the cell. The protein then activates a cascade of signals inside the cell resulting in an increase in the specific growth factor called BDNF, brain-derived neurotrophic factor.
Interestingly, the treatment that produces the largest increase in BDNF isn't a drug at all. It's electroshock therapy.