As we gather with friends and family to celebrate the holidays, it is important to have a strong immunity to avoid getting sick. The extra stress of the season including last-minute shopping, disrupted sleep schedules due to parties and work obligations, and indulging in rich foods can all negatively affect the immune system. Cold weather is also to put an extra strain on our body's natural defenses.

According to Real Simple, nutrition is a major player in boosting immune function. Immune cells need certain nutrients to function properly, says Dr. Gary E. Deng, an integrative medicine specialist at Memorial Sloan Kettering Cancer Center. Eating plant-based foods, foods containing probiotics, and avoiding processed products can help fuel the immune system.

Here are 6 top immune-boosting choices:

• Leafy greens. Aim for at least two cups of leafy greens such as kale, spinach, Swiss chard and arugula to get your share of vitamin C and vitamin K which help promote a healthy immune system. Greens also contain beta-carotene and folate. Use them in salads or whirl in a smoothie.

• Probiotic foods. Probiotic foods such as yogurt, tempeh, kefir and sauerkraut help boost gut health and the “good” bacteria they contain strengthen the immune cells in the intestinal lining, explains Deng. Start your morning with a bowl of yogurt topped with berries and snack on fermented pickles.

• Berries. Susan Levin, the former director of nutrition for the Physicians Committee for Responsible Medicine, advised everyone to “eat the colors of the rainbow” to boost the immune system. “The pigments that give fruits and vegetables their bright colors represent a variety of protective compounds,” she said.  The anthocyanins in blueberries, for example, destroy free radicals, reduce inflammation and boost brain health. Berries are high in vitamin C, a key nutrient that helps the immune system work properly.

• Sweet potatoes. The beta-carotene in sweet potatoes fights cancer and supports the immune system. They also contain more than three times the recommended amount of vitamin A and are a good source of B6, according to AARP. These nutrients are important for the respiratory system, says Anna Taylor, a clinical dietitian at the Cleveland Clinic’s Center for Nutrition, who adds that older adults make fewer immune cells with age so sweet potatoes are an ideal flu-fighter. Taylor suggests roasting them with olive oil, oregano, and black pepper to boost their superpower with immune supportive herbs.

• Green tea. A study published in July in the journal Molecules found strong evidence that this beverage can boost the immune system, says AARP. The catechins in green tea provide antioxidant and immune boosting support.

• Red bell peppers. Keri Glassman, a registered dietitian and founder of Nutritious Life, says she hasn’t had a cold in over a decade thanks to the power of peppers. “Red peppers are one of my favorite foods to incorporate into my diet for immune-boosting benefits,” she says. They are particularly rich in vitamin C, which is important for immunity, reducing the length and severity of colds, and collagen which keeps your skin healthy.