The immune system helps a person battle potentially dangerous bodily invaders such as bacteria and viruses. In some people, however, the immune system overreacts, and the person experiences symptoms after coming into contact with non-harmful substances. Some allergic reactions are minor, while others are more severe. In the novel, Maggie’s reactions are severe for an animal allergy but less severe than other allergies, like some food allergies, which can prove fatal.

Maggie has a severe reaction to animals that have fur or feathers. Her eyes itch, her nose runs, she sneezes, and she develops a strong itch. These are all typical allergy symptoms, but as her doctor tells her, they’re particularly strong for her type of allergy. Many people have much milder reactions to animal dander. The allergy symptoms themselves are caused by histamine, which is a substance that the allergy cells release. This is why people sometimes take antihistamines when they have an allergic reaction: These medications fight the released histamines.

In the novel, Maggie undergoes allergy tests. These tests help determine what a person is allergic to. During the tests, a medical professional marks various parts of the person’s body and then scratches each section with a different allergen. The patient waits a set amount of time, and the medical professional then checks the patient’s reactions.