I think ICT (Indirect Coombs Test ) & DCT (Direct Coombs Test) on serum and RBCs respectively are initially the best markers for evaluating the immunological effects of massive allogenic blood transfusion.
To assess cumulative immunological effects of massive transfusion, you may quantity IgG levels among massive transfusion recipients and a control group, within the population. A comparison of the data generated may be helpful in your research. Thanks
In addition to the red cell antibodies (ICT and DCT as suggested by Dharmesh), you may also want to check HLA antibodies especially if the blood units were not leucodepleted. Good luck.