Question

The immunoglobulin antigenic determinant that changes in the variable region is the?

• A. Idiotype
• B. Isotype
• C. Allotype
• D. Epitope

Hint:

Variable region equals the antigen-binding site, whose unique marker is the idiotype.

Answer 1

The Correct Option is A

Step 1: Immunoglobulin molecules carry three classes of antigenic determinant, defined by where they sit on the molecule.

Step 2: Idiotypes are determinants located in the variable region, specifically in the antigen-binding (hypervariable) site, so they differ from one antibody clone to another. This is exactly what the question asks for.

Step 3: Isotypes are determined by the heavy-chain constant region and define the class (IgG, IgM, etc.); they are the same in all members of a species. Allotypes are allelic variants in the constant region differing between individuals. An epitope is the part of an antigen recognised by an antibody, not a determinant on the antibody itself.

Step 4: Since the variable region determinant is the idiotype, the correct answer is option 1.