What Is Stress? Stress is the body's reaction to harmful situations -- whether they’re real or perceived. When you feel threatened, a chemical reaction occurs in your body that allows you to act in a way to prevent injury. This reaction is known as "fight-or-flight, ” or the stress response. During stress response, your heart rate increases, breathing quickens, muscles tighten, and blood pressurerises. You’ve gotten ready to act. It is how you protect yourself.