Immune Tolerance To Allografts
There are many instance in which an allograft may be accepted without the use of immunosuppressive measure. Obviously, in the case of tissue that lack alloantigen, such as cartilage or heart valve, there is no immunological 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 surveillance. The second is when a state of tolerance has been induced biologically, usually by previous exposure to the antigen of the donor in a manner that cause immune tolerance rather than sensitization in the recipient.
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