T-cell-rich B-cell lymphoma in a cat.

T-cell-rich B-cell lymphoma is a neoplasm recognized in humans in which a neoplastic proliferation of large B lymphocytes is present amid a background of reactive T lymphocytes. A 13-year-old Domestic Shorthair cat developed a mass in the region of the left parotid gland. Histologically, the mass was composed of scattered large atypical cells within a dense background of uniform small lymphoid cells. Immunohistochemically, the large cells were uniformly labeled using antiserum directed against the B-lymphocyte marker BLA.36, whereas labeling of nearly all of the small cells was limited to the T-lymphocyte marker CD3. The histomorphologic and immunohistochemical features of this unique feline neoplasm are characteristic of T-cell-rich B-cell lymphoma of humans.