Final answer:
The specificity of an antibody is due to the variable portions of the heavy and light chains, which form the antigen-binding site.
Step-by-step explanation:
An antibody's specificity is due to the variable portions of the H and L chains. The variable regions of the heavy and light chains form the antigen-binding site on the antibody molecules. The binding specificity of an antibody is determined by the unique patterns of amino acids in these variable regions, which can only bind to target antigens with complementary charges and noncovalent bonds.