Identifying mosaic antigens — which example best fits? Select the best example of a mosaic antigen, i.e., an antigen displaying multiple distinct epitopes across its surface.

Introduction / Context:
The term mosaic antigen describes an antigenic structure that presents many different epitopes, allowing multiple antibodies to bind simultaneously. Understanding this concept helps explain strong agglutination, robust neutralization, and polyvalent vaccine strategies.

Given Data / Assumptions:

• Mosaic antigens have complex, repetitive surfaces.
• Viruses present ordered arrays of proteins on capsids or envelopes.
• Haptens and small peptides typically offer limited epitopic diversity.

Concept / Approach:
Viruses, particularly non-enveloped icosahedral viruses, display repeating capsid protein units that create numerous identical and sometimes distinct epitopes across the surface, a classic mosaic organization. Bacteria also have multiple epitopes but are heterogeneous whole organisms; the simplest, clean example used in immunology texts is viral particles. Haptens are too small and simple to qualify, and a single short peptide offers minimal epitope diversity.

Step-by-Step Solution:

Verification / Alternative check:
Electron micrographs and structural virology demonstrate symmetric arrays of capsid proteins, underpinning polyvalent interactions with antibodies.

Why Other Options Are Wrong:

• Bacterium: complex, but the term mosaic antigen is most classically exemplified by viral capsids.
• Hapten/small peptide: too simple, not mosaic.
• Self lipid: generally non-immunogenic and lacks multiple peptide epitopes.

Common Pitfalls:
Equating organismal complexity (bacteria) with ordered epitope arrays; mosaic refers to the antigenic surface architecture, not organism size.

Final Answer:
Virus