Transplant rejection

When someone receives an organ from another person, their immune system has to settle one question: does this new arrival belong here, or is it from somewhere else? Transplant rejection is what happens when it decides the organ is from somewhere else and attacks it.

And this is not arbitrary. Cells carry proteins on their surface called alloantigens that work like a signature, and that signature differs from one person to the next. The most telling ones are the major histocompatibility complex molecules. T cells, a type of white blood cell, are the ones calling the shots in that response: they read the new organ's signature, see that it is not the one that belongs here, and launch the attack. It is the difference between the key that opens your lock and another one that looks identical but will not go in.

That attack does not always come at the same time, and that is where its three names come from. It can happen within minutes, and then it is called hyperacute. If it appears in the first weeks or months, it is acute. And it can drag on for years, with the immune system attacking the whole time and damaging the organ little by little: that one is chronic.