Identification and role of immunoglobulins in respiratory secretions

The airway surface mucosa begins in the naso-oro pharynx and lines the conducting airways, which include the trachea, major bronchi and bronchial divisions down to the level of the respiratory bronchioles. Although sampling methods used cannot completely isolate these specific areas, analysis of immunoglobulins shows variation especially in the relative proportions of secretory IgA, IgG and IgE. The air exchange surface is sampled by bronchoalveolar lavage, principally, yielding alveolar secretions that contain IgG subclasses, surfactant, etc. and a variety of respiratory cells. Several diseases illustrate abnormalities that can occur in the availability of these immunoglobulins, or in their presumed antibody functions, contributing to human diseases that have a prominent component of infection. Secretory IgA either deficient or degraded by bacterial proteases (especially IgA₁ form) can reduce available anti-viral antibody or perhaps promote colonization of the airways with certain microbes. IgA has a dual role. As it is a important opsonic form of antibody, selective deficiency of IgG, especially subclasses IgG₂ and/or IgG₄, can be associated with recurrent sinopulmonary infections. Sometimes IgG (IgG₄) can be increased in hypersensitivity lung diseases including asthma and orga-antigen induced hypersensitivity pneumonitis (pigeon breeders disease). Finally, cystic fibrosis, affecting the lung with persistent infection often with Pseudomonas aeruginosa, can feature degradation of IgG antibody that may create blocking fragments that impair opsonin-induced phagocytosis, or this antibody can contribute to immune complex formation. Thus, immunoglobulins (antibodies) are an important ingredient of humoral host defenses in the respiratory tract and can contribute to disease, oftne involving infection, if quantitative or qualitative deficiencies in them exist.