P Brandtzaeg
Laboratory for Immunohistochemistry and Immunopathology (LIIPAT), University of Oslo, National Hospital, Norway.

The respiratory mucosa is protected primarily by a secretory immune system that is under complex and only partly understood immunoregulatory control. Secretory immunoglobulins (SIgA and SIgM) protect the mucosal surface by immune exclusion of antigens. However, the fact that most IgA produced in the respiratory tract belongs to the IgA1 subclass renders SIgA in this region susceptible to IgA-specific proteases produced by Haemophilus influenzae, Streptococcus pneumonia, and Neisseria meningitidis. Immunoglobulin G can also perform immune exclusion at respiratory surfaces but, like IgE, it reaches the secretions merely by passive diffusion. The phlogistic properties of antibodies belonging to these classes explain their potential involvement in maintaining mucosal inflammation. In patients with selective IgA deficiency, SIgA is lacking and is not regularly compensated for satisfactorily by SIgM. In such patients unexplained immunoregulatory mechanisms, perhaps involving the local microbiota, give rise to a large number of IgD-producing cells in the upper respiratory tract. Immunoglobulin D cannot act as a secretory antibody and might block the protective properties of IgG; this could explain why these patients are particularly prone to recurrent infections. Our observations show that there are large individual variations in the mucosal immune system with regard to humoral immunity in the upper respiratory tract.

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