Vitamin C is an essential vitamin the human body needs to function. It is a water-soluble vitamin that cannot be made by the body, and must be obtained from foods or other sources. Vitamin C is found in abundance in citrus fruits such as oranges, grapefruit, and lemons, and in green leafy vegetables, tomatoes, potatoes, strawberries, red or green peppers, and cantaloupe. Vitamin C is an antioxidant, a compound that helps block the action of unstable molecules known as free radicals, which can damage cells. Vitamin C is thought by some to enhance the immune system by stimulating the activities of natural killer cells (a type of white blood cell), as well as agents that clean-up ‘sick’ cells.