There are many substances that can naturally boost the effectiveness of the immune system. Some, like certain vitamins and minerals, we have detailed elsewhere, but there are also lots of other compunds, for example phytonutrients (plant extracts) that selectively boost all or part of the immune system, making it more effective.
(Or...how to help your immune system!)
In addition to these naturally-occurring chemicals, there are many things you can do to boost your immune system. Exercise is a good example - many of the hormones and chemicals released by the body in response to exercise enhance the immune system and help protect the body from disease.
Back from immune boosters to nutritional supplements
Essential fatty acids