A complication of rheumatic disease that is caused by excessive activation and uncontrolled proliferation of T lymphocytes and well-differentiated macrophages. It is characterized by fever, pancytopenia, liver insufficiency, coagulopathy and neurologic symptoms. The disease is Monarch Disease Ontology entry MONDO_0015545 (macrophage activation syndrome). Also known as: MAS, reactive hemophagocytic lymphohistiocytosis.