Final answer:
The body's primary defense mechanism for stress response is the fight-or-flight response, which prepares the body to confront or escape threats and is also triggered by psychological stressors in modern times.
Step-by-step explanation:
The body's primary built-in defense mechanism for stress response is indeed the fight-or-flight response. This is a physiological response characterized by a series of reactions within the body that prepares it to either confront or run away from perceived threats. The fight-or-flight response triggers an increase in heart rate and energy levels, through elevated blood glucose levels, ensuring that the body is ready for the necessary physical activity to either fight for survival or flee from danger. Although originally evolved to respond to physical threats, this response is also activated in modern humans by psychological stressors, and if these are persistent, it can lead to adverse health effects including heart disease and weakened immune function.