This process leads eventually to the formation of polyps and obstruction of the ostia of the paranasal sinuses, and to irreversible anti-PD-1 antibody scarring and bronchiectasis. In bronchiectasis,

airway clearance is permanently impaired, perpetuating the vicious cycle of infection and inflammation [5]. Why is Ig replacement effective in preventing pneumonia, while markedly less so in preventing bacterial airway infection in PAD patients? The underlying reason may be that Ig replacement cannot fully substitute for an important part of the physiological airway defence. At the airway surface, the dominant isotype IgG is restricted to the alveolar space where it arrives after passive diffusion from the systemic circulation. Hence, inflammation in the alveolar space, i.e. pneumonia, is effectively prevented by systemic IgG replacement therapy. At the bronchial airway site, as well as in the nasal airways, however, IgA and IgM are the dominant isotypes in the immunocompetent individual. Both isotypes reach the airway lumen by active transport through the epithelium which is initiated by antibody-secreting cells located in the lamina propria of the airways [6]. Patients with primary immunodeficiency (PID) frequently lack both these Ig isotypes and the related antibody-secreting cells. This renders

them susceptible to bacterial and also viral airway infections. Viral infections in turn may predispose to bacterial infection by impairing mucociliary clearance [7], inducing phagocytic dysfunction [8] and/or promote bacterial adhesion [9]. IgA at the luminal site is predominantly polymeric, which leads to differing BI 6727 order immune functions in comparison

to monomeric IgA. Monomeric IgA largely resembles IgG in triggering a proinflammatory response. Polymeric IgA more effectively immobilizes pathogens, prevents their adhesion or binds toxins [10]. These mechanisms allow the removal of pathogens that are inhaled physiologically into the lower airways without causing inflammation, also referred to as immune exclusion [6]. Why is IgA supposed to be an important part of the anti-bacterial airway defence in PAD patients, while apparently the vast majority of individuals with a selected IgA deficiency are not susceptible to prolonged bacterial or viral airway infection? The main reason is probably Buspirone HCl that, in CVID and XLA, patients also lack both IgA and IgM. IgM shares much of the immunological properties of polymeric IgA and may substitute for the lack of IgA in patients with selective IgA deficiency. IgA deficiency was the strongest independent risk factor for bronchiectasis in a prospective study with CVID and XLA patients [4]. While it is widely accepted that Ig replacement therapy is not sufficiently effective in preventing airway disease, it is less clear which measures would ameliorate the disease course in the patients. The true prevalence of chronic sinusitis and bronchiectasis is still unknown.