| Key histologic features |
| Diffuse interstitial infiltration of involved areas. |
| Predominantly alveolar septal distribution. |
| Infiltrates compromised mostly of T and/or B lymphocytes, plasma cells, and macrophages. |
| Lymphoid hyperplasia (mucosa-associated lymphoid tissue (MALT) hyperplasia) - frequent. |
| Pertinent negative findings |
| Lack of tracking along lymphatic routes (bronchovascular bundles, pleura, and interlobular septa), characteristic of lymphomas. |
| Organizing pneumonia, inconspicuous or absent. |
| Lack of Dutcher bodies (ie, intranuclear vacuoles containing IgM monoclonal protein seen in Waldenstrom's macroglobulinemia). |
| Lack of monoclonal light chain staining pattern of plasma cells (polyclonal pattern present). |
| Lack of extensive pleural involvement or lymph node involvement. |
| Lack of necrotizing granulomas. |
