The detection of C4d in a graft biopsy ideally should be amended by clinical information on circulating donor-specific antibodies against major histocompatibility complex (MHC) class I or class II. Therefore, based on our current understanding, C4d accumulation is considered to be a marker for an ‘antibody-mediated allo-response’. Currently, it is unknown whether this lectin pathway plays any pathophysiological role in the activation of C4 in renal transplants. Thus, C4d may also be potentially deposited without prior antibody binding. However, it should be kept in mind that apart from the classical antibody-mediated route of complement activation, C4 can also be activated via an alternative, antibody-independent mechanism, the ‘mannan-binding lectin’ pathway. Since C4d is practically never detected along peritubular capillaries in the native diseased and inflamed kidney, such as active lupus nephritis, antineutrophil cytoplasmic antibody (ANCA) disease or anti-glomerular basement membrane (GBM) disease (personal observation), its detection seems ‘transplant specific’. This observation marks a ‘revolution’: for the first time, a general and robust immunohistochemical marker for humoral rejection is identified. Detection of C4d is regarded as an indirect sign, a ‘footprint’ of an antibody response. Covalent binding renders C4d a stable molecule that can easily be detected by immunohistochemistry ( Figure 1 see Appendix for details of staining protocols). C4d is also found in intracytoplasmic vacuoles of endothelial cells. Following activation and degradation of the C4 molecule, thio-ester groups are exposed which allow transient, covalent binding of the degradation product C4d to endothelial cell surfaces and extracellular matrix components of vascular basement membranes near the sites of C4 activation. Ĭ4d is the degradation product of the activated complement factor C4, a component of the classical complement cascade which is typically initiated by binding of antibodies to specific target molecules. Classification schemes of renal allograft rejection are being revised accordingly. Major attempts are underway to understand ‘C4d-positive humoral rejection episodes’ better. At present, many centres use C4d during the work-up of allograft dysfunction. Basel was the first transplant centre in the world which considered C4d to be a very valuable diagnostic tool and incorporated it into the diagnostic decision-making process. The transplant centre in Basel has gained experience with C4d over the last decade. C4d is regarded as an immunohistochemical marker for a humoral mediated allo-response. Stimulated by the pioneering work by Feucht and colleagues from Munich years ago, C4d has led to major changes in our understanding of kidney transplant pathology. This traditional view currently is under scrutiny. Tubulo-interstitial rejection is a prime example. Consequently, nearly all acute rejection episodes have been classified as ‘cell mediated’. Hence, antibody-mediated rejection episodes frequently remained undiagnosed and unclassified. The difficulties with identifying humoral rejection are due mainly to the lack of typical morphological and immunohistochemical changes characterizing different forms of an antibody response. In particular, the proper identification of humoral rejection episodes after the immediate post-transplantation period causes problems. However, all current classification schemes of renal allograft rejection have major shortcomings. They form the backbone for the clinical decision making, outcome studies and multicentre analyses of the efficacy of new immunosuppressive drugs. Over the past decades, morphological criteria of acute and chronic rejection have been defined, and classification schemes of rejection have been introduced, such as the CCTT and the Banff schemes. The gold standard for the diagnosis of rejection and for guiding patient management is the histological evaluation of a renal allograft biopsy. Antibodies, C4d, diagnosis, rejection, therapy Background