EPO and Immunodeficiency: Secondary PRCA can have various causes (14–16), including autoimmune diseases, lymphoproliferative disorders, hematologic or solid tumors (such as thymomas), immune deficiencies, ABO-incompatible hematopoietic stem cell transplantation, viral or bacterial infections, and medications (e.g., recombinant human erythropoietin, immunosuppressive drugs, antibiotics, and antiviral drugs).