The immune system is made up of cells, tissues, organs, and processes that identify a substance as abnormal or foreign and prevent it from harming the body. Primary defenses include the white blood cells, but skin, mucosa, normal bacteria, enzymes, and proteins also provide protection. During times of stress and malnutrition, immune function may be decreased, meaning that susceptibility to illness is increased. Proper nutrition, including adequate protein, calories, and antioxidants (such as vitamin C, vitamin E, and beta-carotene, which are all found in fruits and vegetables) may help to improve immune response and reduce the risk of illness.
Shils, Maurice, et al. (1999). Modern Nutrition in Health and Disease, 9th edition. Baltimore, MD: Williams & Wilkins.