Alloantigens are most relevant in which event?

• A. Transplantation of a kidney from one individual to another
• B. Vaccination with the polysaccharide coat of a bacterial cell
• C. Oral administration of a live but heat-killed virus particle
• D. Grafting skin from one area of the body to another

Answer: (A)

Alloantigens are antigens that differ between individuals of the same species and are most relevant when tissues are exchanged between different people. In kidney transplantation, the donor’s MHC (HLA) molecules can be recognized as foreign by the recipient’s immune system, triggering T‑cell–mediated rejection due to alloantigen differences. Pathogen-derived antigens, like a bacterial capsule, are not alloantigens. An autograft, such as skin moved within the same person, would share the same antigens and not provoke alloantigen-directed rejection. So the event where alloantigens matter most is transplantation between individuals.