Th17 and Treg distribution in gut biopsies of patients with mild and severe GVHD. Th17 and Treg were identified by the expression of IL-17 or Foxp3, respectively, in CD4⁺ cells using 2-color immunohistochemistry (A-B, IL-17 in brown; C-D, Foxp3 in brown; A-D, CD4 in red). Th17 cells were more numerous in patients with mild GVHD (A arrowhead), whereas Treg were present in patients with severe GVHD (D arrowhead). Counts of double-immunostained cells were independently assessed by 2 pathologists (P.R. and A.J.) on an Olympus AX 70 microscope with wide-field eyepiece number 26.5. At 400× magnification, this wide-field eyepiece provided a field size of 0.344 mm².