In some individuals, the immune system attacks substances such as grass pollen that are usually harmless, resulting in

An allergic reaction

Explanation:

An allergic reaction occurs when our immune system misinterprets a harmless substance into a harmful antigen and initiate an over-expressed immune response against it. Some of these allergic substances are pollen, molds, insect stings, etc.

These allergens trigger the production of IgE antibodies in the body which interact with different cell and initiate the release of chemicals like histamine that cause symptoms of allergic reaction.

It can affect the nose, skin, lung, stomach lining, etc. Therefore the correct answer is an allergic reaction.