Primary Humoral Immunodeficiencies and Bronchiectasis in Adults.

Primary humoral immunodeficiencies are a heterogeneous group of disorders defined by quantitative and/or functional defects in one or more immunoglobulin classes, often with associated cellular immune abnormalities. Their link with bronchiectasis, whose prevalence varies across specific defects, is largely driven by recurrent respiratory infections. Selective Immunoglobulin-(Ig)A deficiency and IgG2 subclass deficiency are the most frequent forms, but common variable immunodeficiency (CVID) is the condition most often associated with bronchiectasis and is usually diagnosed earlier because of its characteristic phenotype. In contrast, the contribution of isolated IgA deficiency or selective IgG subclass deficiencies to bronchiectasis remains controversial. Other reported associations include X-linked agammaglobulinemia, selective IgM or IgG deficiency, and rarer entities such as selective IgE deficiency, unclassified hypogammaglobulinemia, specific antibody deficiency, specific polysaccharide antibody deficiency, and heavy- or light-chain deficiencies. Current bronchiectasis guidelines recommend measurement of serum immunoglobulins and IgG subclasses in patients with compatible features, recurrent infections, or no clear etiology before labeling disease as idiopathic. Identifying immunoglobulin defects is clinically important because they represent treatable traits. The potential role of emerging therapies such as the DPP1 inhibitor brensocatib in immunodeficiency-related bronchiectasis remains uncertain, and ongoing registries will be key to clarifying these relationships.