Tyrosine phosphorylation of STAT6 in response to IL-4. PBMCs obtained from a healthy control subject (A) and from a patient with lymphoma 3 months after transplantation (B) were incubated for 1 hour in medium alone or medium containing 10 ng/mL IL-4 as indicated and analyzed for intracellular expression of phospho-STAT6 by flow cytometry as described in “Patients, materials, and methods.” Logarithm of red fluorescence is displayed on the abscissa and relative cell number on the ordinate. Staining with PE-conjugated anti–phospho-STAT6 mAb is indicated by open histograms and staining with PE-conjugated control mAb by shaded histograms. Expression of phospho-STAT6 after IL-4 stimulation did not differ significantly (P > .4) when comparing control PBMCs (n = 3) with posttransplantation patient PBMCs (n = 6).