The reactive “milieau” in Hodgkin lymphoma. Mast cells and regulatory T cells populate the HL microenvironment showing spatial interaction with RS cells. Immunohistochemical staining for PD-1 and FOXP3 highlights the presence of several regulatory T cells intermingling with RS cells (arrows). Double immunohistochemistry for CD30 (yellow/brown) and mast cell tryptase (purple) shows the tight interaction of mast cells with RS cells.
