Low serum IgM is a marker of ALPS disease activity. (A-B) The serum IgM level (g/L) is inversely correlated with (A) the proportion of circulating DN-Ts (CD3⁺TCRαβ⁺CD4⁻CD8⁻) and (B) the plasma FAS-L concentration (ng/L) measured concomitantly. (C) The serum IgM level (g/L) in asymptomatic MPRs and nonsplenectomized and splenectomized ALPS patients. In relevant patients, IgM levels at the time of sepsis are highlighted in red. The normal range is indicated by the gray zone.