Many predict allergy season this year will be brutal but a U.S. food expert says picking up a few items in the grocery store may help. Phil Lempert, a food industry analyst, trend watcher and creator of supermarketguru.com, says building a strong immune system is one of the best defenses against seasonal allergies and vitamin C -- ascorbic acid -- helps boost immunity. The protective role of vitamin C is vast -- cardiovascular diseases, cancers, joint diseases and cataracts -- are all associated with low levels of vitamin C intake, Lempert says. Vitamin C achieves much of its protective effect by functioning as an antioxidant, mopping up free radicals. Structures that contain fat are also particularly dependent on vitamin C for protection, Lempert explains. The richest sources of vitamin C are sweet peppers, black currants, broccoli, Brussels sprouts, kale and turnip greens but vitamin C is heat sensitive so the longer you cook these vegetables, the more the vitamin decreases so eat as many raw, fresh vegetables, but if cooking, the faster the cooking, the better, Lempert says. Other vegetables high in vitamin C are okra, squash, cabbage, dark leafy greens, cauliflower, peas, sweet potatoes and asparagus. In fruit, prime sources for vitamin C include papayas and mangoes, pineapple, strawberries, citrus fruit, kiwi, cantaloupe, honeydew and watermelon, Lempert added.