Disclosed herein are methods for diagnosing or predicting B-cell rejection in a subject. In one example, for assessing transplant rejection, the method includes determining an antigen presenting index by comparing uptake of a donor antigen to uptake of a reference antigen in a biological sample obtained from the subject. In another example, for assessing GVHD, the method includes determining an antigen presenting index by comparing uptake of a recipient antigen to uptake of a reference antigen in a biological sample obtained from the subject.