Final answer:
A graft from a pig to a human is known as a xenograft, and it generally involves a high risk of rejection due to immune system responses to foreign major histocompatibility complex markers.
Step-by-step explanation:
A graft from one species to another, such as from a pig to a human, is referred to as a xenograft. This process involves transplanting tissues or organs across different species boundaries. Such transplants carry a high risk of rejection due to differences in their major histocompatibility complex markers.
Xenografts are different from allografts, which are transplanted between genetically different individuals within the same species, and autografts, which are transplanted from one area to another within the same individual. Given that xenografts involve cross-species transplantation, there is ongoing research to genetically modify donor animals to reduce the immune response in the recipient, although there are significant challenges to this approach.