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. Author manuscript; available in PMC: 2020 Nov 3.
Published in final edited form as: J Allergy Clin Immunol. 2016 Apr 26;137(6):1674–1680. doi: 10.1016/j.jaci.2016.02.015

Figure 1. Relative concentrations of Ag and Ab determine the roles of IgE and IgG Abs in Ab-mediated anaphylaxis.

Figure 1

IgE-mediated anaphylaxis requires considerably less Ab and Ag than IgG-mediated anaphylaxis. Consequently, when Ab levels are low (A), only IgE-mediated anaphylaxis can occur. When Ag levels are low but Ab levels are high (B), IgG “blocking” Abs prevent IgE-mediated anaphylaxis by intercepting Ag before it can bind to FcεRI-associated IgE and by binding to the inhibitory receptor, FcγRIIB, but the quantity of IgG/Ag complexes is too low to trigger IgG-mediated anaphylaxis. Consequently, anaphylaxis does not occur. When Ag and Ab levels are both high, but Ab is in excess to Ag (C), IgG Abs block the binding of Ag to FcεRI-bound IgE but IgG/Ag complexes can bind to FcγRs; consequently, only IgG-mediated anaphylaxis occurs. When Ag and Ab levels are both high, but Ag is in excess (D), IgG/Ab complexes are sufficient to trigger IgG-mediated anaphylaxis and enough Ag escapes IgG blockade to bind to FcεRI-associated IgE and trigger IgE-mediated anaphylaxis.