An allergen is a substance that can cause an allergic reaction. In some people, their immune system sees this allergen as dangerous. As a result, your immune system reacts by defending against the allergen. 

Allergens can come from many things, including: 

• Food proteins 
• Pollen from plants 
• Dander from pets 
• Ingredients in some medications 

In the United States, the top food allergens are proteins found in peanut, egg, milk, tree nuts, soy, sesame, fish, shellfish, and wheat.