Due to the immediate crisis the brain sets in maximum motion the basics of the body and turns off all the other mechanisms. At that time the heart and the lungs work overtime to send as much oxygen as possible to the bloodstream. Imagine that a dog is barking and chasing you. The muscles respond instantly. Your blood pressure has to be high to deliver the energy all around the body. This is not the time to care about reproduction, tissue repair and digestion. Neither to fight the cold. So, the immune system is disabled, too. Also, there are other observations such as pupil dilation in order to help see in better clarity, increased perspiration to prevent over-heating due to the increased metabolic rate etc. Actually, you don’t care about the long-term project if you are sure it’s long-term.