@indigomoth - Unfortunately, there are all kinds of different reasons why your immune system can't be in hyperdrive all the time. For one thing, it takes energy to maintain it, as well as vitamins and so forth, and the systems that maintain it can wear out.
For another, a revved up immune system can make mistakes. The completary system is said to be involved in causing diseases as well as curing them. For example some forms of arthritis are caused by an overactive immune system. It might also be involved in Alzheimer's disease.
So it's not as simple as that, unfortunately.