International, multicenter, long-term, follow-up study that will enrol HLH participants who have received emapalumab in previous clinical trials, in the context of the clinical development program for emapalumab or under compassionate use (CU).

The aim of this study is to monitor the long-term safety profile of emapalumab in participants who have previously received at least one dose of emapalumab, including survival time after the administration of emapalumab. Moreover, the elimination profile of emapalumab and the immunogenicity will also be assessed. Furthermore, safety, tolerability, efficacy, and pharmacokinetic (PK) profile of emapalumab will be closely monitored in the event that some participants, upon request of the treating physician, will receive emapalumab treatment in the follow-up study.