A heterophile antigen is one that:

• A. Is a self-antigen
• B. Exists in unrelated plants or animals
• C. Has been used previously to stimulate antibody response
• D. Is from the same species but is different from the host

Answer: (B)

Heterophile antigens are shared across different species, so antibodies produced in response to an antigen in one organism can cross-react with antigens from unrelated organisms. This cross-species reactivity underlies how certain serologic tests—for example, those used to diagnose infectious mononucleosis—use antigens from animals to detect antibodies in humans. Because these antigens are present in unrelated plants or animals, they allow antibodies to bind across species, producing a positive reaction even though the organisms are not closely related.

That’s why the statement describing the antigen as existing in unrelated plants or animals best fits. The other ideas describe self-antigens, prior stimulation to evoke an antibody response, or antigens from the same species but different (alloantigens), none of which capture the cross-species nature of heterophile antigens.