A Phase III Clinical Study to Evaluate the Efficacy, Safety, and Tolerability of Plonmarlimab in Subjects With Relapsed/Refractory Rheumatic and Immunologic Disease-associated Haemophagocytic Lymphohistiocytosis (Also Known as Macrophage Activation Syndrome [MAS])

Inclusion Criteria: * Age: 16 to 80 years (inclusive), of any gender. * The subject is willing to participate in this study and voluntarily signs the informed consent form. For minor subjects aged 16 years (inclusive) to less than 18 years, written informed consent must be signed by both the subject and the subject's legal guardian. * Diagnosed with a rheumatic and immunological disease. * Diagnosed with haemophagocytic lymphohistiocytosis (HLH) according to the HLH-2004 diagnostic criteria (excluding molecular diagnosis) * No response to/or dependence on/intolerance to/or worsening during high-dose corticosteroid therapy. High-dose corticosteroid therapy: at least 1.5-2.0 mg/kg/d of prednisone or its equivalent dose for 3 consecutive days, including methylprednisolone pulse therapy (15-30 mg/kg/d, maximum dose 1 g/d, for 3-5 days). Exclusion Criteria: * Confirmed or suspected primary haemophagocytic lymphohistiocytosis (pHLH). * HLH induced by infection (including but not limited to EBV-HLH) or treatment (including but not limited to drugs such as CAR-T cells, TCEs, ADCs). * History of other active neoplasm malignant within 5 years prior to screening, with the exception of successfully treated cutaneous basal cell or squamous cell carcinoma, or localised neoplasms that have been adequately treated with curative intent, including but not limited to, uterine carcinoma in situ, breast cancer in situ, thyroid cancer, etc.; asymptomatic, localised prostate cancer confirmed to …