Which statement best describes the relationship between regular physical activity and immune health?

Prolonged, high-volume endurance training often enhances some immune measures (e.g., natural killer cell activity) and, in many studies, is linked with reduced infection rates.

The relationship between physical activity and immune function is inconsistent across studies; effects vary by exercise dose, population (age, health status), and the immune outcome measured.

Regular moderate exercise lowers chronic inflammation and supports immune defenses.

Rapidly increasing exercise intensity from a sedentary baseline typically produces marked short-term improvements in multiple immune markers compared with staying at low activity.