Abstract: The use for the serum quantification of the cytokine B-Lymphocyte Stimulator (BLyS) for the evaluation of the risk of immunization and transfusion reactions after blood transfusion, and for the monitoring of patients undergoing blood transfusion or re-transfusion in a patient that includes an initial step of taking a sample of blood from the patient, a step of analyzing the blood sample to determine the concentration of cytokine BLyS, a step of comparing the BLyS levels determined in the previous step and one or more reference values of concentration of cytokine BLyS, a step of identifying a significant deviation between the determined concentration of cytokine BLyS and the reference values of concentration of cytokine BLyS indicated in the previous step and a step of assigning a risk and/or therapeutic effectiveness with respect to the immune-mediated transfusion reactions mentioned above, based on the previous steps.