Comparison of Immunoglobulin G Subclass Concentrations in Severe Community-Acquired Pneumonia and Severe Pandemic 2009 Influenza A (H1N1) Infection

We compared immunoglobulin G (IgG) subclasses in patients with severe noninfluenza community-acquired pneumonia (CAP) to those in patients with severe pandemic 2009 influenza (H1N1) virus infection. Low IgG1 and IgG2 levels occurred often in the CAP group; however, H1N1 patients had lower IgG1 and IgG2 levels (5.4 versus 3.3 g/liter [P 0.008] and 2.5 versus 1.2 g/liter [P< 0.001], respectively). Low IgG2 levels may be specifically linked to severe H1N1; however, it is not clear whether this associ-ation is related to H1N1 or to other features of severity. We recently reported an association between pandemic 2009influenza A (H1N1) virus infection severity and immuno-globulinG2 (IgG2) subclass deficiency (5). However, it is not clear whether IgG2 deficiency is unique to severe H1N1 virus infection or whether it is also associated with other forms of severe nonin-fluenza community-acquired pneumonia (CAP). We retrospec-tively assessed IgG subclass concentrations in patients with severe noninfluenza CAP who participated in the Australian CAP Study (ACAPS) (2) and compared this cohort to our original cohort of patients with severe H1N1 virus infection (5)