Stanford Professor Robert Sapolsky's talk about the biological and psychological aspects of depression (https://www.youtube.com/watch?v=NOAgplgTxfc&feature=youtu.be) - well worth the time investment to watch.
A new theory on depression: It's a disease caused by the body's immune system (https://www.theglobeandmail.com/life/health-and-fitness/article-a-new-theory-on-depression-its-a-disease-caused-by-the-bodys-immune/), by Alexandra Shimo, Special to The Globe and Mail, October 2019.
Now a new theory about the cause of depression has emerged: That it is a disease caused by the body's immune system. The idea is that chronic stress causes hormonal dysregulation, and this leads to depression and other inflammatory disorders, such as arthritis, lupus, heart disease and even some forms of cancer.