Final answer:
The D domain is unique to the heavy chain of the polypeptide, contributing to the vast diversity of antigen-binding sites by allowing a larger selection of genetic recombination for B-cell receptors.
Step-by-step explanation:
The domain type that is unique to the heavy chain of the polypeptide is the D domain. Unlike the light chains that contain one variable domain (V) and one constant domain (C), heavy chains contain one variable domain (VH) and a unique D segment that is not found in light chains. The variable region of the B cell receptor (BCR) heavy chain is composed of V, D, and J segments, while the variable region of the BCR light chain has only V and J segments. This allows for a much larger diversity of antigen-binding sites, which are necessary for targeting a wide array of potential pathogens. The constant region of the heavy chain, which can consist of three to four domains depending on the antibody class, determines the immunological mechanism of action of the antibody.