AITL : CD21
Proliferation of follicular dendritic cells (FDC) outside residual follicles is a diagnostic hallmark of angioimmunoblastic T-cell lymphoma (AITL). The FDC meshworks are expanded and disorganized and surround or abut the high endothelial venules - another key histologic feature of AITL. In some cases, the “burned-out” follicles are surrounded by multiple concentric layers of FDC creating an “onion-skin appearance.” The FDC meshwork can be highlighted by immunostains for CD21 (shown here), CD23, or CD35. The CD21+ cells may not be true FDC but activated fibroblastic reticulum cells with upregulation of CD21.