Hypersensitivity pneumonitis, UIP , NSIP, radiation pneumonitis and lymphangitic spread of carcinoma are interstitial diseases. In hypersensitivity pneumonitis, antigen-antibody complexes around the microvasculature cause a neutrophil-rich inflammatory response and subsequent tissue injury. Biopsy in the subacute phase shows heavy infiltrates of lymphocytes and plasma cells in the walls of the alveoli in combination with poorly formed granulomas containing foreign body giant cells. In chronic phases, the interstitial inflammation remains, but fibrosis becomes more apparent and honeycombing can occur. On CT, the alterations in the walls of the alveoli and the inflammation in the interstitium are visible as thickening of the inter-and intralobular lines and thickening of the intralobular interstitium (Fig. 3c) [15] .