Nail biting has been associated with a better functioning immune system because of the exposure to germs on a small scale that your body can build antibodies to.
If a persons body odor smells good to you that means that they have an immune system basically opposite to yours. This happens so the chances of finding a mate with the opposite immune system is greater and the chances of any offspring you produce together will have a stronger immune system.
Scientists think there is a link between our improved sanitary habits and the increase in allergies and auto-immune disorders because our immune systems are looking for something to fight.
If your immune system received access to the inside of your eyes, it would attack and destroy them as invaders. Similarly, if you damage one of your eyes and it isn’t removed and stays in your eye socket your body will make antibodies against it, which will end up attacking and destroying your good eye. …and this sometimes happens.