CD40LG and Systemic capillary leak syndrome: 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.