Concentration dependence of IgE-induced upregulation of FcRIα in basophils cultured for 7 days (n = 6). The abscissa is expressed as the ratio of 22E7 fluorescence for cells incubated with IgE at the various concentrations to cells not incubated with IgE. (•) Data for PS myeloma; (○) data for heat-inactivated PS myeloma. The average starting fluorescence of the basophils was 25 ± 4, corresponding to ≈30,000 receptors per basophil (see Materials and Methods). Basophils were obtained from leukapheresis packs and first downregulated by a 21-day culture.