What characterizes the circulation of defensive Qi in the body?

Defensive Qi, often referred to as "Wei Qi," plays a crucial role in the body's immune system, serving primarily to protect the body from external pathogens. This type of Qi circulates along the surface of the body, regulating the body's defenses against various environmental threats, including viruses, bacteria, and other harmful entities.

Defense mechanisms include the ability to respond swiftly to external invasions, helping to prevent illnesses and maintain health. The characteristic of protective function directly aligns with the role of Defensive Qi, emphasizing its importance in maintaining the body's integrity against external challenges.

While the other options highlight significant functions associated with different aspects of Qi or body functions—such as nourishing organs, maintaining emotional balance, or transforming food into energy—they do not specifically pertain to the primary role of Defensive Qi in protecting the body from pathogens. Therefore, the defining function of Defensive Qi remains its protective capability against external threats.