Brain biopsy of the tumefactive multiple sclerosis lesion. Inflammation (A-D): Haematoxylin-Eosin (H & E) stained section (A) reveals perivascular infiltrate of mononuclear inflammatory cells (blue arrowheads). Many of the inflammatory cells are CD3 positive T cells (B) some of which show spilling in the surrounding neural parenchyma. Fewer perivascular lymphocytes are CD20 positive B cells (C). There are also frequent macrophages around the blood vessels and diffusely in the neural parenchyma (D). The macrophages are intermingled with glial fibrillary acid protein (GFAP) positive stellate astrocytes (inlet in D) and Creutzfeldt cells (inlet in E, black arrowhead). Demyelination (E-H): H & E stained section (E) shows that the margin between the lesion and the surrounding neural parenchyma is relatively sharp (indicated by the dotted line between myelinated (My) and demyelinated (De) regions). Luxol fast blue histochemical preparation (F) confirms almost complete loss of myelin in the lesion, while immunostaining for neurofilaments (G) shows that the axons in the same area are well-preserved. Immunostaining for CD68 (H) further highlights the margin between demyelinated and myelinated areas and reveals numerous foamy macrophages within the demyelinated foci. Scale bar: 100 μm (A-H); 10 μm (inlets in D and E).