Table 1. Histological clues that, when present in the usual interstitial pneumonia pattern, suggest the possibility of an underlying connective tissue disease (CTD) or chronic hypersensitivity pneumonitis (HP).

Feature	CTD	Chronic HP
Cellular (lymphocytic and/or plasmacellular) interstitial infiltrate	+	+
Plasma cells	+	+
Cellular bronchiolitis	+	+
Centrilobular fibrosis, with or without bridging fibrosis between bronchioles and pleura	−	+
Pleuritis	+	−
Small interstitial/peribronchiolar granulomas	–#	+
Coexistence of more than one pattern in the same biopsy	+	−/+

+: positive; –: negative. ^#: except in Sjögren’s syndrome, in which small granulomas can be found.