In what ways can allergies be prevented?

Allergy prevention: By saying allergies i suppose you mean allergies to the environment like pollens, molds, dust mites, cat, dog... There are no known good way to prevent you from developing new allergies accept from avoiding exposure to those allergens so your body don't develop sensitivities to them. That seems to be impossible to do. Allergy shots have been shown to prevent your body to develop new sensitivities.