system
Given the following post, respond as if you're commenting on Reddit:
human
Why is it that even when a donated organ is a match, the recipient still has to go on anti rejection drugs? Furthermore, why is it way harder to find a match for a donor organ than it is for donor blood?
gpt
With blood

 * There is a small number of compatibility factors (blood type and Rhesus +/- factor are the main ones), which means that for any given person, there are many people with the same type of blood
 * Therefore, we can give you blood that is a near-perfect match for your own, so you don't need immunosuppression
 * Blood cells are recycled and have ~120 day life-span, so even if you needed immunosuppression, you'd only need it for a few months.

With all other cells, and therefore all tissue that contains cells (this excludes, for example, ligaments that have been irradiated to destroy donor cells while retaining the connective tissue):

* There are many compatibility factors (called histocompatibility factors), so there is usually nobody who is a perfect match (except for identical twins)
* Therefore, we have to reduce your changes of rejection both by giving you as close a match as we can *and* by suppressing your immune system
* That's why when we say someone's an organ "match", we mean "close enough that immunosuppression therapy can work", not "perfect match".
* And since your transplanted organs are with you forever, you have to be on immunosuppressants forever.