There are times when the body's immune system over-reacts to a situation. That over reaction then causes problems to you.

For example, if there's a waft of some pollen or dust in the air. The normal reaction may be to sneeze a couple of times and that's it.

The over-reactive immune system or one that is hghly sensitized, will sneeze 50 times and the tears would run and the nose would run. That's when you know you have an over active immune system.

Or consider, you have an irritation on the skin - perhaps exposure to a detergent, to urine or something like it. The normal reaction is just that - a minor irritation.

In the case of an over active immune system, the skin (which is the largest immune barrier we have covering us) over reacts and you itch and turn red and it (the skin) may flake or dry and peel off - layer after layer.