An antibody's variable region:
1. Varies in amino acid sequence to allow different antibodies to bind different antigens.
2. Is located in the hinge and stem regions of an antibody.
3. Is a portion of the light chain of an antibody.
4. Is a portion of the heavy chain of an antibody.

• A. 1 & 2.
• B. 1 & 3.
• C. 3 & 4.
• D. 1, 2, & 3.
• E. 1, 3, & 4.
• F. 1, 2, 3, & 4.

Answer: (E)

Verified step by step guidance

1. Understand the structure of an antibody: Antibodies are Y-shaped molecules composed of two heavy chains and two light chains. Each chain has a constant region and a variable region.
2. Identify the role of the variable region: The variable region is responsible for the diversity of antibodies, allowing them to bind specifically to different antigens. This is due to the variation in the amino acid sequence.
3. Locate the variable region: The variable region is found at the tips of the Y-shaped structure, in both the light and heavy chains, forming the antigen-binding site.
4. Evaluate statement 1: It is correct because the variable region varies in amino acid sequence to allow different antibodies to bind different antigens.
5. Evaluate statements 3 and 4: Both are correct because the variable region is a part of both the light and heavy chains of an antibody.