As early as 1977, tracking studies of proteins of varying molecular weights in SCLS sera during flares determined that although albumin (mw 62 kDa) levels decreased by 30%‐50% during an episode, C1 (mw 200 KDa) decreased only 10% and IgM (mw 900 kDa) levels rose in direct proportion to the increase in hematocrit.14 Thus, proteins up to 900 kDa may extravasate during episodes of SCLS. Here, CD40LG is linked to capillary leak syndrome.