Upregulation of FcRIα expression on basophils during a 7-day culture in the absence (○) or presence (•) of 500 ng/mL of IgE (n = 5). Cells used for these experiments had already been cultured for 21 days to downregulate FcRIα. Average starting level of expression was 37 ± 13, which translates to approximately 45,000 FcRIα per basophil (see Materials and Methods). Data are expressed as the increase relative to the day 21 levels of expression (the start of the upregulation phase of the cultures). For cells cultured in the absence of IgE, measurements were made on days 1, 2, and 3 for only three of five experiments (∅); day seven measurements were made for all five experiments. (B and C) Representative flow-cytometric profiles for 1 of the experiments; (B) starting distribution (mean fluorescence, 27), (C) distribution (mean, 86) after 7 days of culture with IgE.