(A) In EBV-HLH, the viral infection causes a hyperinflammatory interaction with T cells and macrophages, resulting in excessive production of interferon-γ (IFNγ), tumor necrosis factor α (TNFα), interleukin-1 (IL-1), and other cytokines leading to HLH. Increased expression of PD-1 renders cytotoxic T cells incapable of controlling the infection (exhaustion). (B) Nivolumab binds to PD-1, rendering T cells competent to control the EBV infection and extinguish HLH.