Why can a transplanted organ be rejected even when donor and recipient blood types match?

Answer: Immune cells recognize other donor-cell markers as foreign

Immune cells recognize other donor-cell markers as foreign ✓ — The immune system can recognize donor tissue markers, including HLA proteins, as foreign even when blood types are compatible. Tissue matching and medicines that suppress immune responses reduce this risk, but blood-type matching alone cannot eliminate it.

Matching blood types leaves incompatible donor blood trapped in the organ — Rejection despite compatible blood types is not explained by trapped blood of an incompatible type. Blood typing checks one set of markers, while immune cells can also react to other markers on the donor tissue.

The organ is too unfamiliar for the body to supply it with blood — A transplanted organ is connected to the recipient’s blood vessels, so unfamiliarity does not itself prevent blood supply. Poor blood flow can injure an organ, but rejection specifically involves an immune attack.