Definition of isoantigens (alloantigens) Which statement correctly describes isoantigens (also called alloantigens) within a species?

Introduction / Context:
Immunology distinguishes antigens by distribution across and within species. Correctly defining isoantigens (alloantigens) is fundamental for understanding blood groups, transplantation, and population genetics. These antigens vary among individuals of the same species and elicit immune responses in nonidentical members.

Given Data / Assumptions:

• Species-level vs. individual-level antigen distribution.
• Clinical relevance in transfusion and graft compatibility.
• Terminology: isoantigen = alloantigen; xenoantigen = heterophilic between species; autoantigen = self-antigen.

Concept / Approach:

Isoantigens are polymorphic antigens present in some, but not all, individuals of the same species. Classic examples include ABO and Rh blood group antigens and histocompatibility antigens (HLA) in humans. Because expression varies, introducing cells or tissues carrying a nonshared isoantigen can provoke an immune response in a recipient lacking that antigen.

Step-by-Step Solution:

Verification / Alternative check:

Transfusion practice requires matching ABO/Rh isoantigens; transplantation compatibility testing matches donor and recipient HLA, directly illustrating isoantigen concepts.

Why Other Options Are Wrong:

• “All members of a species”: That describes species-common antigens, not isoantigens.
• “Different species” options: Refer to xenogenic differences, not alloantigens.
• “Only in plants”: Not relevant; isoantigens are defined across taxa, including humans.

Common Pitfalls:

• Confusing alloantigens with autoantigens (self) or xenoantigens (between species).

Final Answer:

Antigens found in some members of a species