Study to Assess the Efficacy and Safety of Emapalumab in Primary Haemophagocytic Lymphohistiocytosis

Main Inclusion Criteria: * Primary HLH patients with active disease. * Treatment naïve patients or patients having already received HLH conventional therapy, but having not responded, not achieved a satisfactory response or worsened, or reactivated, or are unable to tolerate current standard of care. * Informed consent signed by the patient or by the patient's legally authorized representative. * Received guidance on contraception. Main Exclusion Criteria: * Diagnosis of secondary HLH consequent to a proven rheumatic, metabolic or neoplastic disease. * Active mycobacteria, Histoplasma capsulatum, Shigella, Salmonella, Campylobacter or Leishmania infections. * Evidence of latent tuberculosis. * Presence of malignancy. * Concomitant disease or malformation severely affecting cardiovascular, pulmonary, central nervous system (CNS), liver, or renal function, that in the opinion of the Investigator may significantly affect the likelihood to respond to treatment and/or the assessment of emapalumab safety and/or efficacy. * History of hypersensitivity or allergy to any component of the study regimen. * Receipt of a BCG vaccine within 12 weeks prior to Screening. * Receipt of a live or attenuated-live (other than BCG) vaccine within 6 weeks prior to Screening. * Pregnant or lactating female patients.