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: Antigen Overview:
An antigen is a substance that triggers an adaptive immune response (involving B and T cells). A molecule's antigenicity (or immunogenicity) depends on specific characteristics.
Step 2: Antigenic Properties Explained:
Let's examine the key factors:


A. Size: Molecular weight is key. Molecules below 5,000-10,000 Da are typically weak antigens. Larger molecules are more immunogenic. Haptens, very small molecules, become antigenic only when bound to a larger carrier protein.

B. Structural Complexity: Simple, repetitive polymers (e.g., homopolymers) are poor antigens. Higher structural complexity, like that of proteins with complex tertiary and quaternary structures, increases immunogenicity. Complexity offers diverse shapes (epitopes) for immune recognition.

C. Chemical Nature: Chemical composition matters. Proteins and large polysaccharides are strong antigens. Lipids and nucleic acids are generally weak antigens unless complexed with proteins or polysaccharides.

D. Foreignness to Host: The immune system tolerates "self" molecules. A molecule must be recognized as "non-self" to provoke an immune response. Greater phylogenetic distance between the antigen source and the host increases immunogenicity.

Step 3: Conclusion:
Size, structural complexity, chemical nature, and foreignness are all essential for a molecule to act as an antigen. Therefore, statements A, B, C, and D are all correct.