Complex sentence: Prolonged stress responses may result in chronic suppression of the immune system, leaving the body open to infections, however there is a short boost of the immune system shortly after the fight or flight response has been activated.
Split into simpler sentences: Prolonged stress responses may result in chronic suppression of the immune system, leaving the body open to infections. However, there is a short boost of the immune system shortly after the fight or flight response has been activated.