Question

The ability of a molecule to function as an antigen depends on its:
A. Size
B. Structural complexity
C. Chemical nature
D. Degree of foreign nature to the host
Choose the correct answer from the options given below:

• A. A, B and D only
• B. A, B and C only
• C. A, B, C and D
• D. B, C and D only

Hint:

To remember the properties of a good antigen, think of a "big, complex, foreign protein." \textbf{Big} (size), \textbf{complex} (structure), \textbf{foreign} (foreignness), and usually a \textbf{protein} (chemical nature). All these factors contribute to strong antigenicity.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
An antigen is any substance that can be recognized by the adaptive immune system (B cells, T cells) and provoke an immune response. The property of being antigenic (or immunogenic) depends on several key characteristics of the molecule.
Step 2: Detailed Explanation:
Let's analyze each factor:


A. Size: This is a critical factor. Generally, molecules with a molecular weight less than 5,000-10,000 Da are poor antigens. Larger molecules are more likely to be immunogenic. Very small molecules, called haptens, can only become antigenic when attached to a larger carrier protein.

B. Structural complexity: Simple, repeating polymers (like homopolymers) are poor antigens. A higher degree of structural complexity, such as that found in proteins with complex tertiary and quaternary structures, leads to greater immunogenicity. Complexity provides a variety of shapes (epitopes) for the immune system to recognize.

C. Chemical nature: The chemical composition is important. Proteins and large polysaccharides are the most potent antigens. Lipids and nucleic acids are generally poor antigens unless they are complexed with proteins or polysaccharides.

D. Degree of foreign nature to the host: The immune system is trained to tolerate "self" molecules. Therefore, a molecule must be recognized as "non-self" or foreign to elicit an immune response. The more phylogenetically distant the source of the antigen is from the host, the more potent its immunogenicity.

Step 3: Final Answer:
All four properties—size, structural complexity, chemical nature, and foreignness—are crucial determinants of a molecule's ability to function as an antigen. Therefore, all statements A, B, C, and D are correct.