Measurements of serum IgD concentrations at various ages and in a number of diseases are reported. IgD was not detected in plasma obtained during the first 6 months of life and was only very rarely found in cord plasma. Older children and adults showed a wide range of levels. IgD levels were not increased in three examples of `autoimmune' disease, but increases were noted in some patients having chronic infections. IgD was usually absent in hypogammaglobulinaemia. IgD was not detected in a variety of body fluids.
Lymphoid cells containing IgD were rare in lymph nodes, spleen and intestinal mucosa compared with cells containing other classes of immunoglobulins. In adenoid tissue IgD containing cells occurred more frequently and were often similar in numbers to those containing IgM.