The purpose of this study is to expand the knowledge on the efficacy and safety of emapalumab (previously known as NI-0501) as a treatment for primary haemophagocytic lymphohistiocytosis (HLH) patients, with special focus on long-term outcome and quality of life assessments. Emapalumab can be administered as the first-line therapy, to patients not previously treated with the current standard of care, or can be given to patients who have either failed or were unable to tolerate the available standard of care.