Vitamin C, E, selenium and secondary plant compounds are antioxidants
This works similar to an old trick from Grandma's kitchen: Sprinkle some lemon juice on freshly cut fruit and it won't turn brown. That is because the oxidation process is stopped. Just that here the ingredients in the lemon juice react with air instead of fruit. The lemon juice acts similar to an antioxidant ingredient. That is exactly the process that happens millions of times in the human body. Vitamins protect the body and the cell structures from the free radicals in the air and the immediate environment. The operating principle of the vitamins is similar to that of the lemon juice. They support the cell nutrionally against aggressive, free radicals. Just like the lemon juice, a certain part gets used up and therefore must be consumed in our diet.
Selenium has an antioxidant and immunomodulating effect
Selenium is an essential micronutrient, i.e. it is vital to life and the body depends on nutritional intake. It plays an important role in inactivating free radicals and has various beneficial effects in the immune system area. That is probably the reason why balancing out an increased demand for selenium by substitution has a positive effect on people with immune deficiency problems (e.g. chronic-rheumatic inflammations or cancer). People suffering from chronic-rheumatic inflammations or cancer have a higher demand for antioxidant protective substances which also include selenium.