Antigens are usually ______.

• A. lipids
• B. proteins or polysaccharides
• C. vitamins
• D. nucleic acids

Answer: (B)

Verified step by step guidance

1. Understand the definition of an antigen: an antigen is any molecule or molecular structure that can be recognized by the immune system, specifically by antibodies or T-cell receptors.
2. Recall the common types of biological macromolecules: proteins, polysaccharides, lipids, nucleic acids, and vitamins.
3. Consider which of these macromolecules are most commonly recognized by the immune system as antigens. Proteins and polysaccharides are the primary molecules that serve as antigens because they have complex and variable structures that the immune system can detect.
4. Note that while lipids and nucleic acids are important biological molecules, they are generally not strong antigens by themselves unless combined with proteins or polysaccharides.
5. Conclude that the correct answer is 'proteins or polysaccharides' because these molecules typically elicit an immune response and are recognized as antigens.