In the previous post, we covered the types of rejection that we see in transplanted organs. Obviously, we try to prevent this from happening as far as possible. There are two main ways that we do this.
Firstly, since most of the rejection centres around HLA mismatching, HLA types should be matched so far as possible. The closer the match, the less likely (and less vigorous) the rejection is likely to be. Although HLA matching is definitely first choice, for some transplants (especially the heart and the liver), there is simply not enough time to do this, and so we have to make do.
The other method that is used to prevent rejection is immunosuppressive therapy. As its name suggests, this seeks to dampen the immune response in order to save the transplanted organ. A wide variety of substances are used, including steroids, cyclosporine, azathioprine, antilymphocyte globulins and monoclonal anti-T cell antibodies. Of course, immunosuppressive therapy has the obvious drawback of increasing the recipient's susceptibility to a vast array of microorganisms and cancers. Nonetheless, provided you do transplants judiciously, this risk will still prove less harmful than the lack of a working heart/kidneys/etc. would!
More subtle immunosuppressive techniques are always being investigated, because the ability to transplant organs is primarily limited not by surgical difficultly, but rather by transplant rejection. Of course, stem cells promise a way around this, but this is still a long way off, and another topic completely.
Source: Robbins Pathologic Basis of Disease (6th Edn.), Cotran, Kumar, Collins