Which statement about the variable domains of immunoglobulins is true?

Which statement about the variable domains of immunoglobulins is true?

• A. They represent the complement-binding site.
• B. They occur on both the H and L chains.
• C. They are at the carboxy-terminal ends of the molecules.
• D. They are found only on H chains.

Answer: (B)

Variable domains determine what the antibody binds to and are located at the amino-terminal ends of each chain. An antibody has both heavy and light chains, and each chain contains a variable domain (VH on the heavy chain and VL on the light chain) that come together to form the antigen-binding site. Because these variable regions are present on both chains, the statement is true. The complement-binding site, by contrast, resides in the Fc portion (constant region) and not in the variable domains. The variable domains are at the amino-terminus, not the carboxy-terminus, and they are found on both heavy and light chains, not only on heavy chains.

Variable domains determine what the antibody binds to and are located at the amino-terminal ends of each chain. An antibody has both heavy and light chains, and each chain contains a variable domain (VH on the heavy chain and VL on the light chain) that come together to form the antigen-binding site. Because these variable regions are present on both chains, the statement is true. The complement-binding site, by contrast, resides in the Fc portion (constant region) and not in the variable domains. The variable domains are at the amino-terminus, not the carboxy-terminus, and they are found on both heavy and light chains, not only on heavy chains.