Because of our genetic make up, it is not feasible to simply transplant organs from one animal to another (we are animals after all) since our bodies will see that transplanted organ as a foreign tissue and set forth to destroy it (rejection).
The more compatible two animals are, the less likely that rejection will occur. Even when you transplant across humans, there is need for siginificant medication (with very serious side effects) to suppress the immune system so that it does not reject the transplanted organ. The best transplant is between identical twins, since their genetic make is supposed to be the same (although in reality it may not be entirely so).
Given this, it follows that you cannot simply transplant a pig organ into a human without serious consequences. ( There have been heart transplants in the past from baboons to humans with short term success). The pig is phylogenetically very far removed from humans such that problems of tissue compatibility will preclude simple transplant of organs, although there are certain tissues that you can transplant with some safety. There are thousands of people the world over on a waiting list for organ transplants. They have to wait for donors to become available (living or dead) and many of them die in the process. The quest for artificial organs is on-going but still far away ( see: International Society for Artificial Organs
http://www.isao-society.org)
This is the reason why (for some people) the idea of cloning human tissue is very attractive--- this is a totally different can of worms, but I would like to hear what people think about cloning!. You can use embryonic stem cells to practically create any body organ you want--heart, liver, kidneys etc. and have a sort of spare parts store for people in need of organ transplant. You can even clone your own self for future use-- spare parts etc.
I know how we all feel about the pig. There are many medicines that are derived from animals including pigs ( Insulin for Diabetes used to be one of them), but if that is what we need to stay alive and serve Allah, where is the harm??. This is a very difficult question to answer, and I dont know if we can have a right answer but certainly worth discussing..please chime in.
I apoplogise for rambling on..but I wanted to put in my own 2 cents.
Modibbo