Allergic reactions occur when the body’s immune system responds to normally harmless substances in the environment as if they were toxins. [Page 101]These types of antigens are called allergens and come in many forms, ranging from food to airborne particles. Allergic reactions also vary widely, depending on how the allergen comes into contact with the body. This entry provides an overview of the pathophysiology of allergies, the impact of allergies during various stages of development, and several examples of allergic reactions.

Most allergic responses are Type I (immediate) hypersensitivity reactions. When the body encounters an allergen for the first time, a professional antigen-presenting cell will process the allergen into fragments that a helper T cell, a type of immune cell, can recognize. These T cells then ...