The Unique Molecular Weight of the Heavy Chain from Human IgG3

Abstract

Human IgG consists of four subclasses (IgG1, IgG2, IgG3 and IgG4) which are known to differ from each other in certain antigenic, structural and biologic properties (1–5). In particular, the IgG3 subclass has many unique properties and it is catabolized faster than the other three subclasses (6, 7). Most monoclonal IgG cryoglobulins with anti-IgG activity belong to this subclass (8) as do IgG myeloma proteins that exhibit marked concentration- and temperature-dependent aggregation (9). In addition, IgG3 is the only subclass which does not react with staphyloccal protein A (10). One interesting structural feature unique to IgG3 is five inter-heavy chain disulfide bonds (11). Recently we found that gel filtration of a IgG3 cryoglobulin on a Sephadex (Pharmacia Fine Chemicals, Piscataway, N. J.) G-200 column at pH 3.9 yielded an elution position ahead of human Cohn fraction II (FII).