Oxidants are the most common of the cell damage-causing molecules. Oxidants are produced during healthy cellular metabolism. These normally help modulate inflammation, kill bacteria, detoxify toxins, and maintain a good tone of vascular (blood vessel). Excess, unutilized oxidants damage cell membranes, lipids, and DNA. This damage gets repaired by antioxidants obtained from food.
We get exposed to oxidants from external sources too. Smoking, alcohol, certain drugs, deep-fried and fatty foods are other sources. We get exposed to oxidants through radiation, UV light, heat shock, and strong sunlight too. Other oxidant sources are air fresheners as well as vehicular and industrial pollutants. Oxidants interact with antioxidants, destroying these. Diet will need to have enough surplus antioxidants to counterbalance the loss from oxidants.
Free oxidants that are not neutralized by antioxidants can cause degenerative diseases such as atherosclerotic heart disease, diabetes, skin ailments, arthritis, neurodegenerative disorders, cancers, etc.
Fruits and vegetables are rich in phytochemicals. Some phytochemicals act as antioxidants that will neutralize free oxidants. Phytochemicals help in building body immunity. Three types of phytochemicals have been identified—carotenoids, polyphenols, and allium compounds.
Carotenoids are found in abundance in red, yellow, orange, and dark-green colored fruits and vegetables. Polyphenol-rich foods include herbs, spices, vegetables, green tea, apples, and berries. Allium-rich compounds are garlic, onions, leeks, and related vegetables. Many of these vegetables contain antioxidants such as β carotene, selenium, and vitamins C and E. These chemicals reduce the risk of cell damage.
Vitamins B and D and calcium found in plant and other foods have cancer-preventive properties. Dietary fiber found in whole grain, seeds, legumes, pulses, vegetables, fruit, and products made from these helps in the easy evacuation of stool. Plant-based phytochemical-rich foods help regulate hormones such as estrogen, slow cancer cell growth, and block inflammation.
Cruciferous vegetables such as broccoli, cauliflower, cabbage, related vegetables, lycopene found in tomato products, pink grapefruit, watermelon, and apricots protect several types of cancers.
No single antioxidant molecule has been found to replace the health benefits generated by a combination of natural phytochemicals found in fruits and vegetables. Consumers need to eat 5–10 servings of a wide variety of fruits and vegetables daily to mitigate the risk of chronic diseases and to meet nutrient requirements essential for optimum health (Liu, 2004).
This is just an overview, and there is a lot more to the immunity-building properties of plant-based foods.

Additional reading:

Tri-series on natural solutions for obesity, cancer, and diabetes

Nutrition Facts- a guide to good health

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