There are instances in which an allograft may be accepted without the use of immunosuppressive measures. Obviously, in the case of tissues that lack alloantigens, such as cartilage or heart valves, there is no immunologic barrier to transplantation. However, there are also instances in which the strong predicted response to an allograft does not occur. There are two general cases in which an allograft may be accepted. One is when cells or tissue are grafted to a so-called privileged site that is sequestered from immune-system surveillance. The second is when a state of tolerance has been induced biologically, usually by previous exposure to the antigens of the donor in a manner that causes immune tolerance rather than sensitization in the recipient. Each of these exceptions is considered below.
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