Asthmatic and/or allergic rhinitis ('hay fever') individuals respond differently, but reproducibly, to allergen challenge. Some individuals develop an isolated early response while others also go on to develop a late response. In asthmatic individuals, airway narrowing represents the early phase of the asthmatic response to airway challenge; early phase onset can be detected within ten minutes of allergen inhalation, reaches a maximum within thirty minutes, and typically resolves within three hours. In 50-60% of allergic asthmatic adults, the early response is followed by the late phase asthmatic response, which usually starts between three and four hours after allergen inhalation challenge, and is characterized by cellular inflammation of the airway, increased lung tissue permeability, and mucus secretion. Thus, approximately half of allergic asthmatic subjects are 'dual responders' (developing both an early- and late-phase response following allergen inhalation challenge), 30-40% of allergic asthmatic adults develop an 'isolated early response', and <10% adults show an 'isolated late response'. In any given individual, the pattern of response is generally consistent. In allergic rhinitis individuals challenged by environmental exposure to pollen, equivalent differences exist in the proportions of subjects undergoing isolated early or dual phase nasal responses, with these responses measured by clinical end-points such as common symptoms of allergic rhinitis ('hay fever').