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Immunoglobulin G (IgG) subclass-specific antibody responses were investigated in both human cystic echinococcosis (CE) and alveolar echinococcosis (AE) by enzyme-linked immunosorbent assay (ELISA) and Western blotting. Sera were tested from 81 CE and 50 AE surgically confirmed cases using partially purified hydatid cyst fluid antigen B (EgB), and crude hydatid cyst fluid (EgCF) from Echinococcus granulosus and a protoscolex extract (EmP) prepared from E. multilocularis metacestodes. Eighty-nine percent of CE patients and 94% of AE patients were seropositive for total IgG antibodies. The dominant IgG antibody isotype in both CE and AE was IgG4, for which 73% of CE and 84% of AE cases were seropositive in the ELISA. Anti-Echinococcus IgG1 antibodies were detected in 58% of CE and 84% of AE patients. Sensitivity for IgG2 and IgG3 was significantly lower for both CE and AE patients. The IgG4 antibodies in CE patients predominantly recognized the E. granulosus antigen B subunits, i.e., at 20, 16 and 12 kD, while IgG1 antibodies primarily bound to the 38-kD subunit of antigen 5 from E. granulosus hydatid fluid. The possible functional significance of an IgG4 response and the potential use of combined assessment of total IgG, IgG1, and IgG4 antibodies in serologic diagnosis and follow-up for human echinococcosis are discussed.