Galactose-alpha-1,3-galactose, commonly known as alpha gal and the Galili antigen, is a carbohydrate found in most mammalian cell membranes. It is not found in primates, including humans, who have lost the GGTA1 gene. Their immune systems recognize it as a foreign body and produce xenoreactive immunoglobulin M antibodies, leading to organ rejection after transplantation.