De novo HLA donor specific antibodies (DSA) following transplantation are associated with alloimmune injury and allograft failure. Blood transfusions are allogeneic, and when given post-transplant (PTBT) they may independently increase the risk of HLA antibody development. This study aims to analyse the development of HLA transfusion specific antibodies (TSA) to blood donors of transfusions given post-transplant and examine the impact on clinical outcomes. 244 blood donors of transfusions received by 86 transplant patients (46 who developed a DSA post transfusion and 40 who remained DSA negative) were HLA typed. De novo TSA developed against 150/244(61.5%) blood donors. In 70/150(46.7%) cases the TSA was of shared HLA antibody specificity with a DSA response in the recipient (DSA+=TSA+). This occurred when there was a greater overall HLA match between the blood and transplant donor. DSA+=TSA+ patients had increased risk of allograft failure [p=0·0025] and AMR [p=0·02] compared with the DSA+≠TSA+ patients. To conclude, PTBT may elicit de novo HLA antibodies. Enhanced HLA matching between the blood and transplant donor is more likely to result in a DSA and TSA of shared antibody specificities. Transfusion avoidance or the use of HLA matched or selected blood may reduce this risk and improve outcomes.