Human immunoglobulin G4 (IgG4) antibodies are in many ways unusual. In this review, an overview is given of the structural and functional aspects of IgG4 antibodies, the consequences of IgG4 antibody formation in various disease settings, and the factors involved in the regulation of IgG4 responses. Unlike most IgG antibodies, IgG4 antibodies exist in a dynamic equilibrium with other IgG4 antibodies, continuously exchanging half-molecules resulting in effectively monovalent antibodies that cannot cross-link. Together with the low affinities to C1q and most Fc receptors, and a generally high affinity for antigen, IgG4 antibodies appear to be nature's way of producing "blocking antibodies." On the one hand, IgG4 may contribute to tolerance to allergens, presumably via competition with IgE. Also, IgG4 immune responses to filarial parasites might prevent excessive immune reactions during such infections. On the other hand, IgG4 autoantibodies may be pathogenic, simply because they inhibit the function of their target molecules. Furthermore, IgG4 antibodies to biologicals may result in secondary loss of response. In addition, IgG4 has been implicated to impair humoral immunity to tumors. The role of high IgG4 serum levels in IgG4-related disease has not yet been established. Regulation of IgG4 responses is most likely a multifactorial process, which in vivo requires prolonged or repeated challenge with antigen, and is associated with regulatory T cells, T helper 2 cells, interleukin- (IL-) 4, and IL-10. In vitro, cytokines like IL-4, IL-13, IL-10, and IL-21 have been shown to differentially influence IgG4 production. The properties of IgG4 B cells have now started to be elucidated, and may provide additional clues to explain the unusual dynamics of IgG4-antibody responses.
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