Short answers, pulled from the story.
An allergy is an exaggerated immune response in which the body mistakes an ordinarily harmless allergen for a threat. The mechanism involves immunoglobulin E antibodies binding to an allergen and then to receptors on mast cells or basophils, triggering the release of inflammatory chemicals such as histamine.
The Viennese pediatrician Clemens von Pirquet first used the word allergy in 1906. He coined it after noticing that patients who received injections of horse serum or smallpox vaccine had quicker, more severe reactions to a second injection.
Ninety percent of allergic responses to food are caused by cow's milk, soy, eggs, wheat, peanuts, tree nuts, fish, and shellfish. In the United States population, the most common food allergy is a sensitivity to crustacea, and peanut allergy is not the most common in adults or children.
In the developed world, about 20% of people are affected by allergic rhinitis, food allergy affects 10% of adults and 8% of children, and about 20% have or have had atopic dermatitis. Depending on the country, about 1 to 18% of people have asthma, and anaphylaxis occurs in between 0.05 and 2% of people.
Allergy diagnosis is typically based on a person's medical history, supported by a skin prick test or an allergy blood test that detects allergen-specific IgE. A skin prick reaction usually appears within 30 minutes, and a wheal at least 3 mm larger than the negative control is generally counted as positive.
The hygiene hypothesis holds that insufficient stimulation of the immune system's TH1 arm leads to an overactive TH2 arm that drives allergic disease. It was developed to explain why hay fever and eczema were less common in children from larger families presumed to share more infections through their siblings.
Treatment includes avoiding known allergens and taking medications such as antihistamines and steroids, with injectable adrenaline, or epinephrine, recommended for severe reactions. Allergen immunotherapy, which gradually exposes people to larger amounts of allergen, is useful for conditions like hay fever and reactions to insect bites, but its benefit for food allergies is unclear.