There are several IgG antibody subclasses, including IgG1, IgG2, IgG3, and IgG4.145 People with IgG1 and IgG2 deficiency are susceptible to bacterial capsule polysaccharide antigens such as the pneumonia-causing Streptococcus pneumoniae. The gene discussed is IGHG3; the disease is susceptibility to pneumonia measurement.