What characterizes the immunoglobulin IgD?

IgD is characterized primarily by its presence in very low concentrations within the bloodstream, constituting less than 1% of the total immunoglobulin load. Its primary function is typically related to the immune system, particularly in the activation and regulation of B cells. It is mainly found on the surface of immature B cells, where it plays a critical role in initiating B cell activation upon antigen binding.

The other options relate to immunoglobulins that serve different purposes. For instance, IgG is known as the most abundant immunoglobulin in blood, providing the majority of antibody-based immunity against invading pathogens. Similarly, IgE is well recognized for its significant role in mediating allergic reactions, and IgM is primarily involved in activating complement pathways during the immune response. Hence, the distinct role and presence of IgD in the immune system validate that it encompasses a unique function, thus making it the correct choice to highlight its characterization.