Selective subnormal IgG3 in 121 adult index patients with frequent or severe bacterial respiratory tract infections.

We characterized 121 adults with frequent or severe bacterial respiratory tract infections at diagnosis of selective subnormal IgG3. Mean age was 47 ± 13 (SD)y; 87.6% were women. Associated disorders included: autoimmune conditions 33.1%; hypothyroidism 14.9%; atopy 29.8%; and other allergy manifestations 41.3%. In 34.1%, proportions of protective Streptococcus pneumoniae serotype-specific IgG levels did not increase after polyvalent pneumococcal polysaccharide vaccination. Blood CD19+, CD3+/CD4+, CD3+/CD8+, and CD56+/CD16+ lymphocyte levels were within reference limits in most patients. In regression analyses, independent variables age; sex; autoimmune conditions; hypothyroidism; atopy; allergy manifestations; corticosteroid therapy; and lymphocyte subsets were not significantly associated with IgG subclass, IgA, or IgM levels. Frequencies of HLA haplotypes A01, B08; A02, B14; A02, B15; A02, B44; A02, B57; and A03, B07 were greater in 80 patients than 751 controls. We conclude that subnormal IgG3 and non-protective S. pneumoniae IgG levels contribute to increased susceptibility to respiratory tract infections.