Transplants

Transplantation of skin or whole organs (kidney, hear,
lung, pancreas) has been attempted in humans with varying
success. Transplantation across major hisoincompatibility
barriers, as between different species, is associated with
rapid graft rejection by the recipient. When
transplantation is accomplished between subjects matched
closely for histocompatibility, graft rejection occurs more
slowly. The mechanisms of homograft rejection are complex
and not yet completely understood, but they probably
involve destruction of the grafted cells by recipient cells
sensitized against surface histoincompatibility antigens
present on grafted cell membranes. In addition, in some
instances a graft vs. host reaction may involve damage to
the recipient's cells by the graft cells. Genetic
engineering offers the hope of cures for many inherited
diseases, once the problem of low efficiencies of effective
transfer of genetic material is overcome. 

Another development has been the refinement of the
technique called cloning which produces large numbers of
genetically identical individuals by transplanting whole
cell nuclei. With other techniques, scientists can isolate
sections of DNA representing single genes, determine their
nucleotide sequences, and reproduce them in the laboratory.
This offers the possibility of creating entirely new genes
with commercially or medically desirable properties.