**Core Concept**
Autoantigens are endogenous proteins or molecules that are mistakenly recognized as foreign by the immune system, triggering an autoimmune response. This phenomenon is distinct from the recognition of exogenous antigens, such as those from pathogens or foreign substances.

**Why the Correct Answer is Right**
The correct answer, **D. None**, indicates that neither Blood group antigen nor Forssman antigen is an autoantigen. Blood group antigens are present on the surface of red blood cells and are involved in the ABO blood group system, while Forssman antigen is a heterophile antigen that can cause an immune response in certain individuals. However, these antigens are not considered autoantigens because they are not endogenously produced and do not trigger autoimmune responses.

**Why Each Wrong Option is Incorrect**
**Option A:** Blood group antigen is not an autoantigen because it is a heterophile antigen that is present on the surface of red blood cells and is involved in the ABO blood group system, rather than being an endogenously produced protein or molecule.

**Option B:** Forssman antigen is not an autoantigen because it is a heterophile antigen that can cause an immune response in certain individuals, but it is not an endogenously produced protein or molecule that triggers an autoimmune response.

**Clinical Pearl / High-Yield Fact**
Autoantigens are a key component of autoimmune diseases, such as rheumatoid arthritis, lupus, and type 1 diabetes, where the immune system mistakenly attacks endogenously produced proteins or molecules.

**✓ Correct Answer: D. None**