____ of an allergen is the best way to prevent allergy.

Allergen:

1. Allergens are the substances that cause allergic reactions in the body.
2. Allergens can enter the body by various means like air, food and also skin surface.
3. Generally the first exposure to an allergen does not cause allergy and helps in the formation of antibodies only.
4. But the second or subsequent exposures cause strong allergic reactions.
5. Avoidance to the exposure of an allergen is the best way to prevent allergy.

Final answer: Avoidance of an allergen is the best way to prevent allergy.