Immunoglobulin Secretion in the Airways

Besides the gastrointestinal tract, the epithelial surface of the lung constitutes the next largest area that must be protected from invasion by microbes and toxic agents in the environment. Two principal mechanisms have evolved for the surveillance and protection of this epithelial boundary: One is relatively nonspecific and involves the mucociliary apparatus, which mechanically clears particles from the airways. The second is a more adaptive mechanism and involves the secretory immune system, which often operates in concert with the mucociliary escalator. Well before the recognition of the structure and function of antibodies, it was appreciated that specific neutralizing substances could be found in the secretions of the upper and lower respiratory tract that protected the host from a variety of infectious agents (e.g. influenza virus) (24, 66). Furthermore, these humoral responses to locally applied antigens could occur independently and often in the absence of systemic immunity. The next major advance was the recognition that the antibody responsible for this specific protection along the mucosal barrier was immunoglobulin A, 19A (61).