I was thinking about the girl who had a transplant of the wrong blood type, got another one, but ended up dying. My understanding is that before doctors transplant an organ, they make sure it “matches”. I’m not entirely clear on what constitutes “matching”, but I gather it has something to do proteins and the immune system. What I was wondering is: wouldn’t blood type be part of “matching”? Why would doctors check it separately?
While blood type plays a role in matching tissue types, the main factor in the major histocompatability complex, or MHC molecules. Almost all cells express MHC class I, while a subset of immune cells also express MHC class II. These proteins bind to small peptides (short sequences of protein) and express them at the cell surface where they are recognized by T cells. This is highly simplified, but during an infection, peptides from the pathogen are presented by MHC so that T cells can respond to the infection. Under normal circumstances, the same MHC molecules express bits of normal cell proteins so that the immune system knows that everything is all right.
Humans have 3 different MHC I and 3 different MHC II genes, and there are multiple alleles of each of these genes. This means that there is a huge amount of variation between people, and you have to match all of them. Otherwise, the immune system can recognize that the transplanted tissue is “non-self”, i.e. expressing different MHC as well as foreign peptides. When this happens, both T cells and natural killer (NK) cells attack and destroy the organ, leading to transplant rejection.
Tissue matching does not completely eliminate the possibility of organ rejection, but it reduces the chances of it happening. This is because there are still more factors involved than just MHC, but that’s very complicated and it’s been a while since I did any transplant immunology so I’d probably get it wrong, so I won’t go into that. Unfortunately, if a patient has already rejected one organ, the odds of rejecting a second one are higher because of immune memory as memory cells are ready to respond right away as opposed to a primary response where it takes several days to get T cell activation.