This is often due to the fact that B cells express higher levels of HLA class I than do T cells.10 When class I complement fixing HLA DSAbs are present at a significant level one would expect both the T- and B-cell crossmatches to be positive. A negative B-cell crossmatch in the presence of a positive T-cell crossmatch therefore suggests a technical error. This is not unusual as B cells tend

to be less resilient than T cells and their viability can often be a concern in the assays. These points are summarized in Table 3. Proceeding with a transplant in the setting of a positive T-cell crossmatch, which is not due to an autoantibody, is likely to generate a very poor outcome. In their seminal work in this area Patel and Terasaki described

the outcomes Erlotinib nmr of 30 such transplants.3 Venetoclax Twenty four (24) patients lost their grafts immediately to HAR while another three lost their grafts within 3 months. It is not clear why the other three patients had less severe reactions but it may relate to false positive crossmatches generated by autoantibodies given that DTT was not used in their assays. Other possibilities include false positive tests or lower immunogenicity of the antibodies or antigens in those cases. More recently, a study investigated whether IVIg or plasma exchange was more effective at desensitizing crossmatch-positive recipients so that they might be crossmatch-negative at the time of transplant.11 While most patients were successfully desensitized there was a group of for 10 patients who did not achieve a negative crossmatch but were still transplanted. Of this group 70% developed AMR with 50% losing their grafts. Given this data, even after reducing the antibody titre with a desensitization protocol before transplant, a persistent positive T-cell crossmatch remains an absolute contraindication to transplantation. B-cell CDC crossmatching is not as predictive of HAR as the T-cell CDC crossmatch and there has been much controversy about its role.12 Many centres do not perform B-cell crossmatching for cadaveric renal transplantation because of uncertainty about the significance of a positive result. The major limitation is a rate

of false positive results of up to 50%.13 While a negative result is reassuring a positive result may mean a transplant is cancelled when it was safe to proceed. Another argument against the routine use of B-cell crossmatching is that antibodies to class II antigens are of less significance in generating antibody-mediated rejection. More recently it has been found that they are not so benign.14 B-cell crossmatches are often performed as part of the immunologic assessment before live donor transplantation when there is more time to determine the significance of the result. Paired with information about the presence of DSAbs, determined by more specific means such as antigen-coated beads (Luminex, discussed below) the B-cell CDC crossmatch results may be more meaningful.