Lymphocytic Interstitial Pneumonia,

Low Grade Lymphoma

It is debated whether lymphocytic interstitial pneumonia (LIP) should be distinguished as a benign entity separate from low-grade lymphoma. Both diffuse and localized forms of benign lymphoid proliferations have been described--LIP and nodular lymphoid hyperplasia. In AIDS, LIP is recognized as a lesion with features illustrated below.

Lymphocytic interstitial pneumonia (LIP)

LIP is characterized by:

  • Peribronchiolar (arrow and lower right corner) and perivascular infiltrates of benign-appearing lymphocytes and plasma cells
  • Spread of the same cells into the alveoli
  • Lymphoid follicles--the darker rounded aggregates

Low-grade, small cell lymphoma/Primary pulmonary lymphoma of bronchus-associated lymphoid tissue (BALT) (B-cell type)

Low-grade, B-cell lymphomas are believed to be derived from cells of the marginal zone of bronchus-associated lymphoid tissue. These cell spend their lives in BALT, regional lymph nodes, and the blood where they may visit other mucosa-associated lymphoid tissue before "homing" to BALT (see diagram).


These tumors have an architecture similar to that seen in LIP besides showing areas of confluent growth without necrosis. They are composed of small- or medium-sized lymphocytes, plasma cells, or cells showing features of both (all variants of the same cell type). Immunoglobulin light or heavy chain restriction or gene rearrangement indicating a clonal B-cell population is diagnostic. Morphologic features that suggest lymphoma are:

Lymphoepithelial lesion

Here, numerous lymphoid cells focally infiltrate and destroy bronchiolar epithelium. The arrow indicates the epithelial basement membrane, which is obliterated at the left.

The tumor cells react with B-cell markers. Reactive inflammatory infiltrates are usually T-cell type. The lesions can be delineated better by a stain for keratin that leaves the lymphoepithelial lesions unstained.

Dutcher bodies

These eosinophilic intranuclear bodies (arrows) are suggestive of lymphoma when they are frequent.

Vessel wall invasion

Here, an elastic van Gieson stain shows the elastic tissue (black) of the wall of a venule. Note the subendothelial collections of tumor cells, which narrow the lumen.

Pleural invasion

Here, tumor cells are present focally in the visceral pleura. The arrow marks the position of the inner elastic layer of the pleura.