Histologic appearance of normal lung tissue. L indicates lung alveolae in normal lung. *Normal neutrophil in microvasculature. (B) Lung tissue showing infiltration of alveolae with leukocytes from a patient who died of retinoic acid syndrome. w indicates myeloid cells in the airspace (most leukocytes to left of “w”); the lobated nuclei of myeloid cells is prominently seen. Arrows indicate fibrinous exudate resulting from vascular leak of serum fibrin.