What Effects Do Stressors Like Crowding or Restraint Have on the Immune

What effects do stressors like crowding or restraint have on the immune systems of animals?

A) They can lead to increased levels of immune system activity.
B) They can activate the release of aggressive pheromones.
C) They can reduce various aspects of immune system activity.
D) They can increase levels of serotonin and dopamine.