Immunogenicity — Which statement best defines the property that enables an antigen to elicit an immune response?

Introduction:
Immunogenicity is the capacity of a substance to provoke an adaptive immune response. This question distinguishes correct determinants of immunogenicity from common misconceptions about self antigens, haptens, and antigen types.

Given Data / Assumptions:

• Standard vertebrate adaptive immunity with T-cell help.
• Antigens may be proteins, polysaccharides, or other molecules.
• Processing and presentation via MHC are central for most T-dependent responses.

Concept / Approach:
Effective immunogens are typically foreign, sufficiently large and complex, and able to be processed into peptides that bind MHC for T-cell recognition. Haptens alone are not immunogenic but become so when conjugated to carriers that provide T-cell epitopes. Self antigens are ordinarily non-immunogenic due to tolerance.

Step-by-Step Solution:

Verification / Alternative check:
Carrier–hapten experiments show that small molecules become immunogenic when linked to a protein carrier providing peptides that bind MHC, confirming the centrality of T-cell recognition.

Why Other Options Are Wrong:

• Usually a property of self antigens: false under normal tolerance.
• Never a property of antibodies: false; foreign immunoglobulins are immunogenic across species.
• Never associated with haptens even with carriers: false; hapten–carrier conjugates are classic immunogens.
• Only carbohydrates: false; proteins are often the most potent immunogens.

Common Pitfalls:
Confusing antigenicity (ability to bind antibodies) with immunogenicity (ability to induce an immune response).

Final Answer:
It depends on the ability of the native antigen to be processed and presented by MHC to T cells.