Alveolar Histiocytosis

Aggregates of macrophages within the subpleural alveo­lar spaces are seen occasionally in the lungs of older laboratory rats. On gross examination, they usually appear as dull pale yellow foci (Fig. 2.54), and become accentuated when lungs are fixed in formalin. On histo­pathology, there are intra-alveolar collections macro­phages with abundant foamy cytoplasm. This lesion is common in many species of animals.