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BACKGROUND: Pathogen reduction methods have the potential to modify blood components, resulting in immunologic reactions or compromised blood components. This study evaluated the hypothesis that there is no immune response to riboflavin-and-ultraviolet [UV]-light–treated red blood cells (RBCs), as observed by serology and by survival of RBCs in circulation. STUDY DESIGN AND METHODS: Three baboons were in each treatment group: 1) untreated (negative control), 2) quinacrine mustard (QM)-treated (positive control), and 3) riboflavin-and-UV light–treated (test group) RBCs. In the immunization phase, autologous test or control RBCs were injected subcutaneously on Days 0, 21, 42, and 49. Plasma samples from these days were tested against test or control RBCs by flow cytometry and standard serology. On Day 56, autologous 51Cr-labeled test or control RBCs were injected. Blood samples were taken over 21 days after injection to determine RBC survival (t1/2). RESULTS: Untreated and riboflavin-and-UV-light–treated RBCs showed no evidence of significant immunoglobulin G (IgG) binding after incubation with autologous plasma. RBC-bound IgG was detected on QM-treated RBCs after incubation with autologous plasma. This antibody was inhibited by QM, as demonstrated by a hapten inhibition study. t1/2 values for the untreated and riboflavin-and-UV-light–treated RBCs were 7.3 ± 0.8 and 7.5 ± 1.7 days, respectively; the t1/2 value for QM-treated RBCs was 2.3 ± 2.9 days. CONCLUSION: Treatment with riboflavin and UV light did not render RBCs immunogenic. Positive controls indicated that immunization promoted an immune response. In the 51Cr-labeled RBC survival phase of the study, riboflavin-and-UV-light–treated RBCs exhibited behavior similar to negative control RBCs. Detrimental immunologic or functional side effects were not observed.