Some individuals with IgG subclass deficiencies are asymptomatic. Others with IgG subclass deficiencies are prone to recurrent sinopulmonary infections. Such infection-prone patients exhibit reduced antibody responses to bacterial capsular polysaccharides. Defective anti-polysaccharide antibody responses are most often seen in individuals with IgG2 subclass deficiency with or without concomittant IgA deficiency. The molecular mechanisms underlying igG subclass deficiencies are unknown.