Disclosed herein are methods for diagnosing or predicting acute cellular and/or humoral rejection in a subject. In one example, a method of assessing organ rejection includes contacting a first sample comprising antigen presenting cells (APCs) obtained from a subject in need of or having received an organ transplant with a donor antigen from a donor under conditions sufficient to induce uptake of the donor antigen contacting a second sample comprising APCs obtained from the subject in need of or having received an organ transplant with a third-party antigen under conditions sufficient to induce uptake of the third-party antigen and determining an antigen presenting index by determining a ratio of uptake of the donor antigen in the first sample to uptake of the third-party antigen in the second sample, wherein the ratio of greater than one indicates organ rejection and the APCs are monocytes or monocyte-derived cells.
