What is Macrophage activation syndrome?

Macrophage activation syndrome is a severe, potentially life-threatening, complication of several chronic rheumatic diseases of childhood. It occurs most commonly with systemic-onset juvenile idiopathic arthritis. In addition, MAS has been described in association with systemic lupus erythematosus , Kawasaki disease, and adult-onset Still's disease. It is thought to be closely related and pathophysiologically very similar to reactive hemophagocytic lymphohistiocytosis. The incidence of MAS is unknown as there is a wide spectrum of clinical manifestations, and episodes may remain unrecognized.