Allergy is a systemic disorder that can affect the nose, lungs, eyes, skin, and gastrointestinal tract, simultaneously or in succession, during the course of a patient's life. The reason for the variety in clinical expression of allergic disease is not known. Genetic predisposition, as well as environmental factors, contribute to the development of the allergic phenotype. Local and systemic inflammatory processes also seem to play a role; however, their exact contribution to the clinical manifestation of airway allergy still remains to be clarified. Although it is clear that allergen exposure of a sensitized target organ is associated with immunopathologic changes at the level of other target organs, the mechanisms underlying this relationship are far from being resolved. Today, most data suggest a systemic link between mucosal sites, involving the bloodstream, bone marrow, and mucosa-associated lymphoid tissue. This review will focus on the interaction between the nose and lungs in allergic disease and the possible implications for the gastrointestinal mucosa.