Atopy or atopic syndrome is a predisposition toward developing certain allergic hypersensitivity reactions. Atopy may have a hereditary component, although contact with the allergen must occur before the hypersensitivity reaction can develop. The distinction between atopy and atopic disease is important. A child with atopy produces specific IgE antibodies after exposure to common environmental allergens and is said to be sensitised to that allergen. The presence of specific IgE antibodies is measured by means of a skin prick test or radioallergosorbent testing (RAST). Eczema, asthma and rhinoconjunctivitis are clinical syndromes each defined by a collection of symptoms and signs and are commonly referred to as the atopic diseases. While most children with these conditions are atopic, some are not, and, conversely, some children with atopy may not manifest atopic disease.

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