The immune system is on charge of the protection of our body against the various agents it identifies as possible aggressors; whether they are viruses, bacteria, allergens, toxins, malignant cells, etc. Nutrition, as we will see, plays a very important role in this, and for that reason the term immunonutrition has been coined to refer to the science involved in studying the relationship between nutrition and immunity. Immune system This system is responsible for providing our bodies with the necessary defence mechanisms. The skin, mucous membranes and mucus secretions are good examples of the physical, chemical and biological barriers that the body erects against possible attacks. There are two types of immune response: innate or non-specific; such as inflammation and the barriers that we have mentioned, and specific; acquired or adaptive. These are the type of responses that the body continues to develop as a defence mechanism as it comes into contact with certain substances or microorganisms. The role of immunonutrition Immunonutrition is the science responsible for the study of the relationship between nutrients and immunity. It does it by observing how the nutrients present in food influence the immune response the body develops against what it identifies as potential aggressors and studying the immunological markers associated with the nutritional status. Nutritional status and essential nutrients The immune system works adequately and efficiently when a person is healthy and well nourished. Poor nutrition will trigger a weakening of the protection mechanisms, in such a way that the skin and mucous membranes can be affected in malnourished patients, reducing their ability to function and their effectiveness. There are also specific nutrients that play an important role in the immune system and which can be vital for people that suffer from particular health problems. Essential nutrient An essential nutrient is one which we must include in our food to cover the needs of the body, because the endogenous synthesis (i.e. our own body's production of that substance) is insufficient. In certain circumstances, some non-essential nutrients are converted into essentials, as the amount that the body needs cannot be sufficiently synthesised. This is what happens, for example, with glutamine in the case of critically ill patients. Nutrients and immunity As we have said, there are a variety of nutrients directly involved in the working of the immune system. Let's look at the main ones: Vitamins and minerals

• Vitamin A: Necessary for the production of antibodies and to help maintain healthy skin and eyes, and the digestive and urogenital system. We find vitamin A and its precursors in foods like carrots, apricots, spinach, butternut squash, sweet potato, paprika, parsley, eggs, baby eels, etc.
• Vitamin D: Regulates the immune function through its role in the production of antibacterial proteins. Vitamin D is largely found in oily fish, eggs, mushrooms and full-fat dairy products.
• Vitamin E: Has both an antioxidant function and the specific function of increasing the activity of T-cells, protecting the cell walls. It can be found in oils (such as sunflower and soybean), peanuts and other nuts, corn, wheatgerm, etc.
• Vitamin C: As well as being an antioxidant, it also regulates the immune response and stimulates the functioning of white blood cells. Vitamin C can be found in parsley, peppers, broccoli, Brussels sprouts, kiwi, papaya, strawberries, lemon, etc. But we should bear in mind that it is a vitamin which is destroyed by heat and so cooking damages its structure.
• Copper: This mineral forms part of the innate immune response, but is difficult to calculate copper's organic concentration and the relationship between dosage/immunity. The most copper-rich foods are pulses, whole grains, cocoa, walnuts, raisins, prunes, sesame, pistachios, etc.
• Selenium: Provides an antioxidant action and aids the immune system, however, if taken in excess may have a proinflammatory effect, for which reason when taken as a supplement the dosage needs to be carefully measured. Selenium can be found naturally in Brazil nuts, whole grains, seafood and fish.
• Iron: Assists with the production of T-cells and forms part of the antioxidant enzymes, but also generates reactive oxygen species that contribute to oxidation. It is also vital to ensure the correct amount of iron is taken in supplements. The foods richest in iron are bivalve molluscs (like cockles, clams and mussels), meat and vegetables (such as soybeans and other pulses), nuts, sesame, etc. Its chemical form in vegetables is, however, less bioavailable.
• Zinc: This mineral has an immunomodulating effect and has been used in paediatrics to counter respiratory infections and to help the immune system recover in cases of malnourishment. It can be found in pine nuts, sesame, sunflower seeds, wheatgerm, cheese, meat, pulses, etc.

 As we have seen, although vitamins and minerals have provided benefits in certain cases, it has not been possible to establish the exact dosages required, for which reason further research into the effects of these nutrients is required. Other immunostimulants

• Proteins: The amount and type of proteins consumed in a person's diet have been seen to have an influence on their immune response.
• Glutamine: Protects and restores the damage caused in the intestinal lining due to certain treatments of health issues. A dosage of a glutamine supplement of at least 0.2 g/Kg weight/day has been shown to have positive effects on the intestinal lining of those patients receiving parenteral nutrition and also a variety of positive effects on patients post-surgery.
• Omega-3 fatty acids: These limit the proinflammatory effect of Omega-6, and their use in nutritional formulas has demonstrated positive results of various kinds, among them, an improved immune response.
• Arginine: When it is under stress the body does not produce or synthesis enough arginine. This amino acid improves the cellular immune function and the healing of wounds. An arginine supplement of up to 30g a day has been shown to have benefits, although further studies are considered necessary to fully determine its effectiveness.
• Nucleotides: These molecules need to be taken in sufficient quantities, through the food we eat, to maintain an adequate functioning of the immune system, but there is insufficient evidence to recommend their use as supplements.

All of these nutrients are normally used within a clinical setting to supplement the food given to sick people experiencing a high degree of physical stress due to their illnesses and/or the treatment they are receiving. Further research is needed on the use of each of these elements in each particular type of situation.