Macrophage Activation Syndrome.

Macrophage activation syndrome (MAS) is a state of immune hyperactivation that can result in life-threatening multisystem end-organ dysfunction. Often termed a "cytokine storm," MAS occurs among the rheumatic diseases most typically in Still's disease but also in systemic lupus erythematosus and Kawasaki disease. MAS can also accompany infection, malignancy, and inborn errors of immunity. This review provides a practical, evidence-based guide to the understanding, recognition, and management of MAS in children and adults, with a primary focus on MAS complicating Still's disease.