In general, vitamins, regulate biochemical reactions in your body. They act like the key in the ignition of a car. Without them, the reactions can't get started. Some vitamins have antioxidant activity. They form the structures in the body, e.g., calcium in bone, iron in the blood, and they also regulate metabolic processes.
Minerals also participate in antioxidation.
Free radicals damage cells, including T-cells, and are caused by infections such as HIV, smoking, bad fats, and pollutants such as exhaust fumes in the air. Some vitamins are antioxidants. These are plant compounds, many of which are antioxidants, that seem to help prevent heart disease and cancer. Like antioxidants, they too protect cells. Many plants that contain phytochemicals are brightly colored, such as red tomatoes.