Monarch Disease Ontology entry MONDO_0018304 (Schnitzler syndrome) is a rare, underdiagnosed disorder in adults characterized by recurrent febrile rash, bone and/or joint pain, enlarged lymph nodes, fatigue, a monoclonal IgM component, leukocytosis and systemic inflammatory response. Also known as: chronic urticaria with gammopathy, chronic urticaria with macroglobulinemia.