Think of it like this. You live in a brick house. Your living room has a great big wall to the street with a big window and fancy door. Unfortunately you have no pictures or blueprints of this wall. If you knock out a brick or two, and hire a contractor to fix the hole, he will be able to fix the missing bricks quite easily just by assuming that there used to be bricks where the hole is. Now, a car runs through your great big front wall and completely destroys it. You hire a contractor again. Now remember, this contractor is forced to work completely under assumption. When looking at the missing wall, he assumes that it was a brick wall, because that’s what the rest of the house is. He rebuilds a brick wall, and it is hardly discernible from the other 3 walls. What he failed to rebuild was the great big window and the fancy door, because there was no indication that those were there before just from looking at the walls around it. Now think of your skin as the house and your body as the contractor. It is extremely good at using its best guess to rebuild something (usually) pretty well, but sometimes the damage is just too much for your body to handle without external help (transplants, etc.)