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Not getting enough vitamin D can weaken your immune system and lower your lymphocytes. Make sure you're getting at least 600 IU of vitamin D every day.You may not be able to get all of your vitamin D from diet alone. Talk to your doctor about taking vitamin D supplements. Vitamin E supports your body's production of B-cells and natural killer cells. To get the benefit, you will need to take between 100 to 400 milligrams per day. People who are generally healthy need less, while people who are less healthy may require more.  Since vitamin E is a fat soluble vitamin, you should take it with a meal that contains at least 3 grams (0.11 oz) of fat. If you want to eat your vitamin E, great options include sunflower seeds, almonds, spinach, safflower oil, beet greens, canned pumpkin, red peppers, asparagus, collard greens, mango, avocado, and peanut butter.  You can find vitamin E supplements at drug stores, vitamin stores, and online. Selenium helps your body produce more white blood cells. Since you may not get it easily in your diet, selenium can be taken as a supplement. When taken along with zinc, both minerals are more effective at supporting your immune function.  The recommended daily allowance of selenium for adults is 55 mcg per day. If you are pregnant, you should aim for 60 mcg, while nursing women should consume 70 mcg. You can also eat your selenium if you like to consume a lot of seafood. It's present in foods like oysters, crabs, and tuna.

Summary:
Include vitamin D in your diet. Try vitamin E. Add selenium.