I read your post with interest. I'm also very sorry that you have this particular affliction. I don't think you need worry about explaining depression to anyone. I don't think it's possible. When I'm not in a depressed state depression is just an abstract concept... it isn't something that someone who isn't depressed could ever truly comprehend. It's kind of like acute physical pain... you know it's bad but unless you actually feel acute pain you don't really get it. If you did... you'd be in acute pain.
Anyway... I can relate on some level. I first felt depression at about the age of three, but I don't think mine is biological. It may seem egotistical but I think I just see the world more clearly or in a bigger picture... and humanity just doesn't meet my hopes and expectations.
I would not even presume to know how to offer you support or comfort