We tend to think of most allergic conditions as isolated diseases, for example rhinitis (nose), asthma (lungs) or urticaria (skin). But many patients with asthma have allergic rhinitis and vice versa. The hypothesis of "united airway" may extend beyond the airway according to the Norwegian researchers who think that allergy may be a systemic disease.
They found an association between adolescent allergic organ involvements (lung, nose and skin) and reporting of other health problems (headache, muscle pain and abdominal pain). According to the study authors, allergy may be expressed beyond organs commonly viewed as part of an allergic disease, and hence may support a hypothesis of a systemic nature of allergic diseases.
Allergy: a systemic disease? The HUNT and Young-HUNT study, Norway. Elin Tollefsen, Arnulf Langhammer, Leif Bjermer, Pål Romundstad, Turid Lingaas Holmen (2008). Pediatric Allergy and Immunology.