An IgD immunoglobulin complex, circulating in human. Circulating IgD is composed of two identical immunoglobulin heavy chains of the IgD isotype and two identical immunoglobulin light chains (kappa or lambda), held together by disulfide bonds, and present in the extracellular space, in mucosal areas or other tissues, or circulating in the blood or lymph. [ http://www.ncbi.nlm.nih.gov/pubmed/11282392 https://www.worldcat.org/search?q=bn%3A0781765196 ]