Cortisol's purpose.

It is proposed that cortisol's primary purpose is to mobilize the body's defenses against water-losing intestinal diseases and corticosterone's against serum diseases. They do this by inversely controlling those immune cells, enzymes, and hormones, etc. that affect survival during infection. These glucocorticosteroids affect fight-or-flight mobilization as an adjunct made possible because most processes that enhance immunity have a reverse effect on fight-or-flight.