Why do immune system rejects a transplant?

The immune system recognizes the transplanted item as a foreign object that doesn't belong in the body and tries to destroy it.The same way it tries to destroy germs.There are drugs called immunosuppressants that lower the immune system allowing for many transplants to be successful.However this leaves the patient more vunerable to diseases.

The immune system can recognize the cells of a transplanted organ and if doesnt match it will always rejects the process.

The organs we currently use for transplants are not created from stem cells. They come from someone who has passed, was an organ donar and their cells were the best match for the person needing a transplant. The drugs given help the patient's body not reject the organ. This works for some patients but not all.

your body doesn't know what it is, so it thinks that it needs to fight it off, since it is not your DNA. So that's why they put you on a immune system suppressant, maybe something like Cellcept. That way your Immune system can't attack it, you would be on it for a while until it seems like your doing better. But it just don't take some times.

immune system can recognize the type of blood if your donor match with your blood type and tissue type, both types of immune system response are involved.

If the donor and recipients dont match it will automatically rejects the process.

if the donor and recipients dont match automaticallly your immune system will reject it.

Its a very long and complex answer, so i'll send you in the right direction:

why do we have different blood types, and what makes them different.

everyone is different