New information is developing so rapidly in the entire field of immunology that one is unable to remain abreast of all advancing fronts. In many cases, consider able information has accumulated as the result of the efforts of many investigators, but the conclusions from the various laboratories have not been summarized recently in a comprehensible manner. One such situation has to do with work on IgD. An up-to-date report on this immunoglobulin was included in Volume 10f this series, but since that time there has been considerable progress in the deter mination of its structure and function....