Several immune system cell types and processes have been recently identified in muscle in inclusion body myositis, dermatomyositis, and polymyositis. Plasmacytoid dendritic cells, the immune system's professional producer of the type 1 interferons alpha and beta, are present in dermatomyositis muscle and skin. These tissues also show high expression of transcripts and proteins from genes that are induced by interferon alpha and beta. Myeloid dendritic cells, which contribute to an immunologic synapse responsible for activation of the adaptive immune system, are abundant within muscle in inclusion body myositis and polymyositis. B cells and plasma cells, effector cells of the humoral immune system, have been stimulated by antigen to transcribe immunoglobulins and produce antibodies in muscle in all three of these diseases. The presence of these immune cells and processes suggests revisions in models of the pathogenesis of the inflammatory myopathies and provides rationales for future therapeutic approaches.