Two-parameter analysis of HbF and TfR in density-fractionated sickle cells. F cell analysis is on the x-axis, with positive and negative regions determined by the nadir between the two peaks. The positive region for transferrin receptor was determined with the aid of matched nonspecific IgG (idiotype control). The regions containing T⁺F⁻ and T⁺F⁺ cells are indicated. (A) Fraction I, idiotype control. (B) Fraction I, anti-TfR. (C) Fraction II, idiotype control. (D) Fraction II, anti-TfR. (E) Fraction III, idiotype control. (F ) Fraction III, anti-TfR.