Antigenic component of an individual’s tissues which may be a target of autoimmune responses by autoreactive B cells (autoantibodies) or T cells, including proteins (e.g., enzymes, structural proteins), glycoproteins (e.g., 2-glycoprotein I), nucleic acids, (e.g., double-stranded DNA), phospholipids (e.g., cardiolipin), and glycosphingolipids (e.g., gangliosides).