15 Foods That Boost the Immune System Immune system boosters Feeding your body certain foods may help keep your immune system strong. If you're looking for ways to prevent winter colds and the flu, your first step should be a visit to your local grocery store. Plan your meals to include these 15 powerful immune system boosters. 1. Citrus fruits Most people turn to vitamin C after they've caught a cold. That’s because it helps build up your immune system. Vitamin C is thought to increase the production of white blood cells. These are key to fighting infections. Popular citrus fruits include: grapefruit oranges tangerines lemons limes clementines Because your body doesn't produce or store it, you need daily vitamin C for continued health. Almost all citrus fruits are high in vitamin C. With such a variety to choose from, it's easy to add a squeeze of this vitamin to any meal. 2. Red bell peppers If you think citrus fruits have the most vitamin C of any fr