Gross and histologic evidence of aGVHD. (A-I) Subject 1. Photographs of skin lesions (A-C) show palmar and solar erythema, and edema and bullous lesions in the lower extremities. Photomicrographs (all 200×) of hematoxylin and eosin (H&E) (D), anti-CD3 (E), and anti-CD56 (F) stained sections of skin involved with GVHD. There is basal vacuolization of the epidermis, apoptosis of epidermal keratinocytes, and infiltration of T cells and NK cells around dermal vessels and along the dermal–epidermal junction. Exocytosis—infiltration of lymphocytes into the epidermis—is also seen. (G-I) Photographs taken during flexible sigmoidoscopy that demonstrate severely congested, erythematous, eroded, granular, and inflamed mucosa in the rectum and distal sigmoid colon. Colonic biopsy (I) shows apoptotic crypt injury and crypt loss (H&E, ×200). (J-M) Subject 2. Photograph from colonoscopy (J) demonstrates congestion and mildly erythematous rectal mucosa. Colonic biopsy photomicrographs of H&E (K, ×400), anti-CD3 (L, ×600), and anti-CD56 (M, ×600). Crypt injury with epithelial apoptosis and crypt loss is observed, with infiltration of T and NK cells within and around injured crypts.