how and why severe exercise decrease the immune system?

Answer 1

i would guess that it is a form of stress, so it would boost cortisol ( which decreases your immune response)

Answer 2

Primarily because strenuous exercise releases cortisol which suppresses the immune system. Immune system suppression can last up to 72 hours after exercise. Also smaller factors such as release of free radicals from damaged muscle tissue and the acute inflammatory response of the body to the tissue damage play a minor role. This is why some athletes take antioxidants to help their body recover. The cortisol not only suppresses the immune system, but also breaks down muscle which works against the work you just did. Insulin can counter the effect of cortisol, which is also released during exercise. Proper nutrition will help your body recover itself.