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Introduction Selective IgM deficiency (SIGMD) remains a poorly described primary immunodeficiency. The condition is frequently under-recognized and no genetic defect has been identified to date. Case Description We present two patients with gradually progressive chronic cough and dyspnea, unresponsive to multiple courses of antibiotics and steroids. Both were non-smokers with no significant past medical history, exposures or reflux. Chest radiographs and pulmonary function testing were unremarkable. Case 1: 60-year-old lady with a high-resolution chest computed tomographic (CT) scan revealing cylindrical bronchiectasis (Figure 1, panel A). Immunoglobulin assays revealed low IgM (9 mg/dL; ref 37-286 mg/dL). She has had one bronchiectasis flare since diagnosis. Case 2: 75-year-old gentleman with chest CT showing bronchiectasis (Figure 1, panel B) and low IgM (18mg/dL; ref 37-286 mg/dL). He too has had only one bronchiectasis flare in the last year. Both patients had IgA, IgG, and IgE levels within normal limits and protective titers against protein and polysaccharide vaccines. T, B, NK cell flow cytometry was unremarkable. Since diagnosis, they have been evaluated with immunoglobulin assays and screening for autoimmune disease and lymphoma twice yearly. Neither has required prophylactic antibiotics. Discussion SIGMD has a prevalence of 0.03 – 0.4% in the general population. SIGMD is not considered a classic etiologic factor for bronchiectasis and SIGMD diagnosis rates of around 14% have been reported in bronchiectasis patient cohorts. In prospective studies evaluating phenotypes of bronchiectasis patients, only 2 in 189 patients had SIGMD. However, lower IgM levels were noted consistently among these patients, raising the question of a potential association.
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