Immunohistochemical staining for CD4, CD8, MHC-I and MHC-II in skeletal muscle of aged, adult and young cows. Aged cows (Group A): there is a mild to moderate inflammatory infiltrate in the endomysium consisting mostly in CD8+, rare CD4+ T cells. Scattered muscle fibers show sarcolemmal immunolabeling for MHC-I and MHC-II antibodies. HRP method with Mayer’s hematoxylin counterstain. Adult cows (Group B): there is a mild inflammatory infiltrate in the endomysium consisting mostly in CD8+, rare CD4+ T cells. Scattered muscle fibers show sarcolemmal immunolabeling for MHC-I, but not for MHC-II antibody. HRP method with Mayer’s hematoxylin counterstain. Young cows (Group C): no inflammation is detectable and there is absence of MHC-I and MHC-II immunolabeling on muscle fibers. HRP method with Mayer’s haematoxylin counterstain.