Zinc: Found in foods such as beef, lamb, capsicum, egg yolks, sunflower and pumpkin seeds, ginger, oysters and seafood, zinc is essential for a healthy immune system. Signs of a zinc deficiency can include white spots on finger nails, poor wound healing, loss of sense of smell and taste, acne, and frequently catching colds and stomach bugs.
Vitamin C: It is important to eat plenty of foods that are high in vitamin C, but they are better eaten raw because cooking destroys most of the Vitamin C content, as does freezing. Apart from boosting the immune system, Vitamin C is important for collagen production, 
healthy bones, teeth and gums, it is an antioxidant and helps with iron absorption. Some of the symptoms of Vitamin C deficiency include bleeding gums, poor wound healing, bruising easily and high cholesterol. Foods high in Vitamin C include capsicum, broccoli, blackcurrants, kiwi fruit, paw paw, pineapple, rosehip, tomatoes, sweet potato and citrus fruits.
Vitamin A: Another important vitamin for immune health, Vitamin A enhances white blood cell function. The white blood cells, also known as leukocytes, are the cells that fight infection. Vitamin A is found in foods such as carrots, egg yolk, green leafy vegetables, sweet potatoes, liver and mint. Some symptoms of a Vitamin A deficiency include acne, allergies, poor sense of taste and smell, sinus problems and poor immunity.
B Vitamins: Found in red meat, asparagus, nuts, eggs, avocado, legumes, sardines and salmon, the B Vitamins (especially B1, B2 and B5) are
another important immune system vitamin. Signs of a deficiency can include constipation, poor immunity, insomnia, cracked skin at the corners of the mouth, digestive disturbances and dermatitis.
Vitamin E: Essential to the immune system, Vitamin E is found in foods such as almonds, egg yolk, hazel nuts, beef, corn and sunflower seeds. Interestingly, mega high doses of Vitamin E can be effective in treating autoimmune conditions like Rheumatoid Arthritis and Lupus by actually suppressing the immune system. Never, under any circumstances self medicate with Vitamin E if you have an autoimmune condition.
Vitamin D: Vitamin D plays a big role in the absorption of Calcium and 
is vital for bone health. It is also very important for a strong immune system. Most of our Vitamin D is synthesised from the sun, but low Vitamin D levels are very common. It can also be obtained from fish liver oils, egg yolk, butter and sprouted seeds.
Iron: Iron deficiency can paralyse the immune response, so it is important to keep iron levels up. In saying that, it is also very important to ensure that your iron levels don’t go too high as this can have a negative effect on your immune system. Foods high in iron include liver, avocado, 
almonds, oysters, pine nuts, parsley, sunflower and pumpkin seeds. Symptoms of iron deficiency can include tiredness, poor wound healing, brittle nails, constipation, digestive disturbances and a frequent need to clear the throat.
Selenium: Found in brazil nuts, cashews, celery, eggs, fish, garlic, liver, broccoli and alfalfa, Selenium is a mineral that people are often deficient in, due to mass farming practices which deplete the soil of minerals. The importance of Selenium in immune function is demonstrated by the fact that adequate levels of Selenium is the single most important nutrient factor for survival of people with AIDS. As with all the functions of the body, there are other nutrients that are important for a healthy immune system, but the ones above are generally classed as the most important ones.
Other ways to boost your immune function:
- Ensure you get adequate sleep. Sleep is the time that your body rejuvenates itself, so without adequate sleep – around 7-8 hours each night for adults – your body is not able to fully rejuvenate.
- Reduce your stress levels. Although short term stress can actually help with your immune system, long term stress can be very damaging. When stress occurs, the body releases the hormones adrenaline and cortisol. These hormones increase the heart rate and release glucose into the blood stream for extra energy to run away from whatever “predator” is threatening you. The problem is that the cortisol also curbs the non essential functions of the body, like the immune and digestive systems. This is not a problem during short periods of stress but prolonged stress curbs these systems long term, resulting in a depressed immune system.
So make sure you allow time to relax and de-stress yourself. Deep breathe, meditate, take up a sport, yoga or just spend some time outside, enjoying nature.
- Reduce, and preferably cut out completely, the sugar in your diet.
Sugar suppresses your immune system. Consuming 6 teaspoons of sugar can decrease your immune function by 25% for 12 hours and consuming 25 teaspoons of sugar by 95% for 12 hours. There are many other advantages of reducing sugar, including lowering your LDL cholesterol, lowering your triglycerides, reducing weight, lowering your chance of Type 2 Diabetes, improving liver function, and reducing the possibility of certain cancers.
- Maintain good gut health. 80% of your immune system is located in your gut, so maintaining good gut health by ensuring that the good bacteria outnumber the bad bacteria is essential. This can be done by reducing (and preferably eliminating) sugar and refined carbohydrates in your diet and by introducing fermented foods like miso, sauerkraut, kefir and tempeh into your diet. Don’t take antibiotics unless it is absolutely essential, and always take probiotics afterwards. Also every few months, it is a good idea to take a course of probiotics.
So, if your immune system is letting you down, try eating more of the foods mentioned in this article, and look after your gut and your emotional health. Do not self diagnose vitamin and mineral deficiencies, and do not self prescribe supplements. Like everything in life, too much is not necessarily a good thing and harm can be done by taking vitamin and mineral supplements if you are not deficient. If you have concerns about your immune system, get help from a qualified practitioner.
By Andrea Southern, Naturopath, Clinical Nutritionist, Herbalist.
References:
http://drhoffman.com/article/nutrients-that-boost-immunity-2/ http://articles.mercola.com/sites/articles/archive/2009/04/18/probiotics-the-case-for-healthy-bowels.aspx http://www.livestrong.com/article/270454-what-vitamins-help-boost-your-immune-system/ http://www.garvan.org.au/news-events/news/new-findings-connect-diet-and-intestinal-bacteria-with-healthier-immune-systems http://www.sciencedaily.com/releases/2013/02/130201100149.htm