Muscle biopsy with routine histochemistry. Muscle biopsy specimen stained with hematoxylin-eosin (A, B) and Gomori trichrome (C, D). Extensive inflammatory cellular infiltrations are evident in the endomysial and perivascular space. There are also marked variations in fiber size and the prolif-eration of connective tissue. White bars indicate 100 μm.

Muscle biopsy with immunohistochemistry. Cellular composition of inflammatory infiltrates in the patient's biceps muscle (A–E). Staining with hematoxylin-eosin (A) and immunostaining for CD8⁺ T cells (B), CD4⁺ T cells (C), B (CD20) cells (D), and macrophages (CD68) (E). CD4⁺ T cells and macrophages are mainly evident. A few CD8⁺ T cell are also scattered in the endomysial spaces, but they do not surround or invade any normal-appearing muscle fibers, while normal looking-muscle fibers are focally surrounded and invaded by CD8⁺ T cells in polymyositis (F). Immunohistochemical localization of MHC class I (G). MHC class I reactivity is evident on the sarcolemma of muscle fibers as well as in the extracellular matrix and endothelium. Immunohistochemical localization of MAC deposits (H–J). MAC deposits are evident within necrotic fibers (H) but not on vessel walls (I), However, in dermatomyositis (J), the vessels including endomysial capillaries are strongly stained with MAC. The black bar indicates 100 μm. MHC, major histocom-patibility complex; MAC, membrane attack complex.