Macrophage activation syndrome (MAS) is a potentially lethal complication of systemic juvenile arthritis, a form of arthritis that can occur with unremitting fevers and rashes, and can be severe and debilitating. T cells in the blood of children with sJIA/MAS may be abnormally activated to cause inflammation. We propose to look more closely at these cells to understand how inflammation develops in sJIA/MAS patients. I will collect blood from children with sJIA (with or without concomitant MAS) and healthy children. I will extract cells, stain them with a special color that labels them and sort these cells based on their color. Then I will look at how these cells interact with different proteins to see if there is any difference in healthy children. Our hope is that by understanding how T cells may cause inflammation in sJIA/MAS, we can create new medicines to treat sJIA and MAS.