One of the best things to give your dog to boost their immune system is omega-3 fatty acids. Research shows that omega-3 fatty acids help to reduce inflammation. More recently it was found that DHA-rich fish oil enhances the activity of white blood cells. However, be aware of the potential side effects of omega-3 fatty acids for dogs. These include altered platelet function, impaired wound healing, weight gain, and an upset stomach. It also has the potential to cause insulin sensitivity. As always, it's best to check in with your vet before giving your pup any new supplements in their diet!