The precipitin-type model-a first attempt to explain FcεRI dose-response behavior. (A) A sub-optimal dose of multivalent Ag would only cross-link a minor fraction of IgE-bound FcεRIs and result in only weak downstream signaling and induction of effector functions. (B) An equivalent Ag concentration optimally would cross-link all IgE-bound receptors and cause maximal activation of Ag-triggered signaling pathways and effector functions. (C) High, supra-optimal concentrations of multivalent Ag were initially thought to result in the formation of monovalent complexes with every IgE binding “its own” Ag. Thus, cross-linking of IgE-bound FcεRI should not occur. Comparable to the sub-optimal situation only weak signaling and execution of effector functions would be the outcome. Of note, this model of FcεRI activation based on the precipitin curve of in-solution immunoprecipitation is not supported by experimental data (see text).