Answer: Angioimmunoblastic T cell lymphoma

Histology: Histologic sections show a node with diffuse architectural effacement. Cortex is compressed to a few areas on the peripheral, and the lesion appears to expand paracortex. There is a prominent vascular proliferation, and the infiltrate is mixed, comprising small lymphocytes, histiocytes, plasma cells and larger immunoblasts. A histologic feature characteristic of this entity is the preservation of the subcapsular sinus, with a “skip lesion” extending outside the capsule. Immunohistochemical findings in this case are classic. Although CD3+ T cells are not overwhelmingly increased, they are the dominant cell type.

There is expression of CD10 on many of these cells.

Nodules of CD20+ B cells are present compressed at the superficial part of the lymph node cortex, while there are also scattered B immunoblasts within paracortex as well.

These are EBV positive.

Finally, there is a prominent increase in CD21+ follicular dendritic cells, not merely reflecting residual follicles, but actually representing an expanded FDC population in paracortex.

Histologically, the differential diagnosis might include Hodgkin lymphoma, T cell rich large B cell lymphoma, or possibly an unusual reactive process, but the immunophenotyping results permit only one diagnosis.

Discussion: AILT is considered to be a tumor of the CD4+ T cells of the germinal center, and the immunophenotype reflects this, as evidenced by expression of CD10, normally seen on germinal center B cells, as well as the striking increase in FDCs that accompany the tumor. Both B and T-immunoblasts may be seen; the former are frequently EBV positive as was seen in this case. They are not part of the neoplastic process, though progression to diffuse large B cell lymphoma has been reported.