Oily fish like salmon, tuna and mackerel are high in omega-3 fatty acids, which can help fight harmful inflammation. One study in Brain, Behavior and Immunity found that omega-3s may help to reduce anxiety as well—this can also strengthen the immune system.
Its cloves contain allicin, a sulfuric compound that produces antioxidants. A study in Advances in Therapy found that those who took garlic supplements between November and February had fewer colds than those who didn't. They also tended to get better faster if they did get sick.
Researchers at Cornell University found that wild blueberries had the most antioxidants of any fresh fruit, out of the ones they tested. Try adding some to a breakfast smoothie or sprinkling them on your oatmeal.