I heard they are humanized 'pigs'. Ie. engineered so their organs don't trigger our immune system. Not sure how true is this, or it is just a myth.
https://www.wnycstudios.org/podcasts/radiolab/articles/alpha... and https://www.wnycstudios.org/podcasts/radiolab/articles/retur... are good listens on the subject.
In particular from the second one:
> SOREN: Which brings me back to that little part of the article that piqued my interest in the beginning, because the pig that they got the kidney from to do this was a very special pig. Normally, pigs and other mammals that aren't primates have a sugar in their body that our bodies don't have, and so we don't like it or see it as foreign. And that's why usually an organ from another animal would get rejected. But this pig had been genetically modified. It had had the gene that makes that sugar removed, so it didn't have that sugar. Which is part of the reason this worked. And that sugar just so happens to be called "alpha-gal."
In the second segment (linked by GP), they first re-played the previous segment (also linked by GP) about alpha-gal allergy. They then revealed that the company behind the alpha-gal safe pigs (which are being grown for medial research purposes) heard the previous one about the alpha-gal allergy, and after hearing it decided to seek FDA approval for alpha-gal safe pigs in the food market (which they received).
I think O&C and The Year of the Flood represent the most likely near-future world, maybe 2035 or so, of any science fiction I can name.
"More than 95% of human anti-pig antibodies are directed against 3 pig carbohydrates: Gal 1 to 3αGal (≈80%–90%), Neu5Gc (≈5%–15%), and β4Gal (1%–5%)"
"To reduce anti-pig antibody binding to the xenograft, the 3 principal carbohydrate antigen targets have each been successfully removed from pigs by gene knockout (KO) (Figure 3)"
Progress Toward Cardiac Xenotransplantation https://www.ncbi.nlm.nih.gov/labs/pmc/articles/PMC7990044/