FIGURE 16-1 People in the United States living with functioning organ grafts, 1999-2007. (Data from OPTN/SRTR Annual Report 2009. Available at: Accessed April 2010.)

experimental results were recapitulated in clinical transplantation. Perhaps the most compelling evidence showing that allograft rejection is an adaptive immune response was the finding that the ability to rapidly reject a transplant can be transferred with lymphocytes from a sensitized to a naive host.

Transplant immunologists have developed a special vocabulary to describe the kinds of cells and tissues encountered in the transplant setting. A graft transplanted from one individual to the same individual is called an autologous graft. A graft transplanted between two genetically identical or syngeneic individuals is called a syngeneic graft. A graft transplanted between two genetically different individuals of the same species is called an allogeneic graft (or allograft). A graft transplanted between individuals of different species is called a xenogeneic graft (or xenograft). The molecules that are recognized as foreign on allografts are called alloan-tigens, and those on xenografts are called xenoanti-gens. The lymphocytes and antibodies that react with alloantigens or xenoantigens are described as being alloreactive or xenoreactive, respectively.

The immunology of transplantation is important for several reasons. First, immunologic rejection remains one of the major problems in clinical transplantation. Second, although transplantation of tissues is not a normal phenomenon, the immune response to allogeneic molecules has been a useful model for studying the mechanisms of lymphocyte activation. Third, many immunosuppressive therapies that have proved to be useful for a variety of immunologic and inflammatory diseases were first tested and shown to be effective for treatment of graft rejection, which is a clinically important immunologic reaction that can be measured rapidly and with precision. Most of this chapter focuses on allogeneic transplantation because it is far more commonly practiced and better understood than xenogeneic transplantation, which is discussed briefly at the end of the chapter. We consider both the basic immunology and some aspects of the clinical practice of transplantation. We conclude the chapter with a discussion of hematopoietic stem cell transplantation, which raises special issues not usually encountered with solid organ transplants.

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