What physiological system is primarily affected by stress, leading to increased susceptibility to illness?

The immune system is primarily affected by stress, which can lead to increased susceptibility to illness. When an individual experiences stress, the body responds by activating the hypothalamic-pituitary-adrenal (HPA) axis, resulting in the release of stress hormones such as cortisol. While cortisol has important functions, prolonged exposure to elevated levels can suppress the immune response, making the body less effective at fighting off infections and diseases. This immunosuppressive effect can lead to a higher likelihood of illness, highlighting the significant relationship between stress and the immune system.

In contrast, while the musculoskeletal, cardiovascular, and gastrointestinal systems can also be impacted by stress, the immune system is particularly crucial in determining overall health and vulnerability to infections. Stress can exacerbate conditions in these other systems, but the direct relationship with immune function is what primarily ties stress to susceptibility to illness.