This shows that stress reduces the activity of the immune system, making them more susceptible to becoming ill. Advantages of the study are that blood tests are objective measurements, so there was no social desirability or experimenter bias when analysing the results. The participants were also compared to themselves which removes participant variables on how they perceive stressors. The experiment was a field experiment so there was no manipulating of the independant variable to deliberately cause stress to the participants which makes it ethical. It also means the research has ecological validity and can be applied to the wider world.
However, a field experiment may have extraneous variables which impact the results. For example, the student lifestyle of having little sleep and poor accomidation may have been an alternative source of stress. Another disadvantage is that there was a biased sample used of students. It may not be possible to generalise students response to stress compared to the general population. Riley also did a study using rats on a rotating turntable for five hours. He found that over time, their white blood cell activity dramatically decreased and when injected with carcinogens they developed tumours more rapidly than a control group of rats. This showed that stress can make us reduce our immune systems activity which can cause diseases to worsen or progress more rapidly. However, as the research was conducted on rats it can be argued that it cant be applied to humans as out stress response may differ.