Why can transplanted cells from another person trigger immune rejection?

In the field of regenerative medicine, transplanted cells from another person (allogeneic cells) can trigger immune rejection because the recipient’s immune system recognizes them as "nonself" rather than "self."12

This recognition is driven by the presence of specific molecules on the surface of the donor cells called antigens, specifically the major histocompatibility complex (MHC), also known as human leukocyte antigens (HLA) in humans. Because these antigens are highly variable between individuals, the recipient's T lymphocytes perceive the donor cells as foreign invaders.134

Once identified, the immune system initiates a complex response to destroy the graft:
* Cell-mediated immunity: T cells directly latch onto the foreign antigens and stimulate the infiltration and destruction of the transplanted tissue.1
* Antibody-mediated response: B lymphocytes may produce antibodies that weaken or destroy the foreign cells, a process that is particularly rapid if the recipient has been previously sensitized to the donor's antigens.15

To mitigate these risks, regenerative medicine prioritizes the use of autogeneic (the patient's own) cells whenever possible.26