CD38 and macrophage activation syndrome: To investigate how the expansion of CD38+ HLA-DR+ T cells and NK cells in SD-associated MAS compares with other inflammatory diseases, we examined patients with Kawasaki disease (n = 10), acute viral infection (n = 18), MIS-C (n = 20), juvenile dermatomyositis (JDM) (n = 11), SLE (n = 9), and MAS secondary to other causes (n = 5).