Can a human be naturally resistant from allergies and sickness?

I have no allergies from any kind of plants, I only sneeze from dust but..who doesn't...I am not allergic to any types of food, or medicine. I have no allergies based on animals either. I've been around my dad who had the swine flu (but not anymore), and people who have gotten rid of it, but I never caught it. When people around me have a cold or the flu I don't catch anything. Is there something in my body that is naturally defending me from such things?