MUSK and thrombotic thrombocytopenic purpura: Despite their immunological inertness, several well-described autoimmune diseases are caused by IgG4 subclass autoantibodies, such as pemphigus, muscle-specific kinase (MuSK) myasthenia gravis (MG), and thrombotic thrombocytopenic purpura (TTP), and in recent years the number of potential IgG4 autoimmune diseases has rapidly grown.