Allergic rhinitis is a well-defined disease. Especially seasonal allergic rhinitis is well suited for studies of pathogenesis and pathophysiology, as there are plenty of patients and as allergen challenge, outside the pollen season, can produce an experimental disease state with inflammation in the nasal mucous membrane. In addition, the nose is easily accessible for direct inspection, sampling of secretions and lavage fluids, and brush and mucosal biopsies. In recent years many researchers have taken advantages of these possibilities, and as a consequence, our knowledge of the pathogenesis and pathophysiology of allergic rhinitis has increased considerably (1–3).