You can add a dog food supplement to your dog's diet to increase immunity and help him fight off illness. Some good supplements include: 1  Neem oil, which boosts the production of T cells, a vital part of your dog's immune system.2  Vitamin C is antiviral and antibiotic.3  Omega fatty acids make your dog's digestion more efficient and can increase his energy levels. 4  They are good for his hair, skin, eyes, and internal organs.ome good supplements include: 1  Neem oil, which boosts the production of T cells, a vital part of your dog's immune system. 2  Vitamin C is antiviral and antibiotic. 3  Omega fatty acids make your dog's digestion more efficient and can increase his energy levels.