Aside from being rich in vitamin C, which is the number one vitamin that boosts immune function, Spinach is also packed with antioxidants and beta carotene that are significant in the infection-fighting capability of our immune system. You can add spinach to your smoothie ingredients, use as a base for salads, add to omelettes or just lightly cook it and add it to other cooked dishes. Make sure to only cook lightly to retain all the nutrients.