IgG subclass antibody production in human serum sickness

The role of IgG-subclass antibodies in the spectrum of immunologic disorders has not yet been fully defined. In an attempt to understand its role in an immune complex-mediated disease, we studied patients who developed serum sickness (SSX) after treatment with an equine-derived immunoglobulin, antithymocyte globulin (ATG), for bone marrow failure. The predominant IgG subclass produced was IgG1, representing nearly 80% of all IgG anti-ATG activity present. The appearance of IgG anti-ATG antibodies and C1q-containing immune complexes was closely correlated with symptoms of SSX. Although other antibody isotypes were present and may have contributed to the patients' symptoms, it is evident that IgG1 is the predominant IgG subclass produced in human SSX caused by a heterologous protein.