CD200 and Immunodeficiency: The highest expression of exhaustion markers (PD-1, PD-L1, CTLA-4) and immunosuppressive molecules (CD200/CD200R) was observed in the SID EBV+ group compared to all other groups, including SID EBV−, which may reflect the enhanced mechanisms of immunosuppression and lymphocyte exhaustion accompanying EBV reactivation in the course of secondary immunodeficiencies.