The three ways in which a recipient can reject their transplanted organ are shown in this slide.
Hyperacute rejection is due to pre-existing antibodies to HLA and ABO antigens. This occurs very quickly after transplantation and usually results in death of the graft and patient. We are able to screen for the existence of these antibodies before the transplantation by taking a sample of blood from the donor and recipient. Therefore this type of rejection rarely occurs.
Acute rejection is mediated by T cells, macrophages and antibodies. This usually occurs after the first week and before the end of the first year. We fight against this by using medications that limit the T cell and antibody response but as of yet we have not been able to discover a medication against macrophage responses.
Chronic rejection is characterized by fibrosis of the organ. This may occur after the first year of transplantation. The real cause of chronic rejection is unknown but it is possible that it is mediated by the innate immune system, and in particular the macrophages found in the innate immune system.