The nose is the airconditioner of the airways. Because normal breathing is through the nose, most airborne particles are filtered there; hence the nasal mucosa is the first line of defence against particles in the air. Pathogenic and non-pathogenic antigens continuously bombard the epithelium of the nasal airway. These antigens are mainly removed non-immunologically by the first defence layer of the mucosa, consisting of mucus, ciliated epithelial cells, glycoproteins/lysosymes. If the antigen passes this defence layer, specific and non-specific immunological defence mechanisms exist. The non-specific defence consists of phagocyting cells like neutrophils and macrophages and the complement activation. The specific defence mechanism (resulting in a specific immunological reaction in relation to a certain antigen) is formed by the antibodies, mainly secretory IgA and to a lesser extent IgG and immunocompetent cells in the nasal mucosa. Activation of the specific defence mechanisms may lead to inflammation which can be allergic. The intense co-operation of mechanical, aspecific and specific immunological defence results in a tightly controlled balance between a proper defence against pathogens and hypersensitivity. Failures in these defence mechanisms, or their co-operation, results in upper respiratory infection and/or allergy.