Final answer:
The antibodies that always exist as monomers on the surface of B cells are IgD. They serve as antigen-binding receptors and aid in B cell maturation, but are not secreted in significant quantities.
Step-by-step explanation:
The antibodies found on the surface of B cells, which always exist as monomers, are IgD.
B cells express membrane-bound immunoglobulins that serve as antigen-binding receptors, critical for initiating the immune response. IgM and IgD are both found on the surface of naive B cells, but IgD is the immunoglobulin that always exists as a monomer on B cells.
Unlike IgM, which can form a pentamer upon secretion, IgD is typically not secreted and remains membrane-bound, with only trace amounts found in serum. These trace amounts likely originate from the degradation of aged B cells. As part of the panel of human immunoglobulins, which also includes IgG, IgA, and IgE, IgD plays a specific role in antigen recognition and the maturation of B cells into plasma cells.