SUMMARYImmunoglobulin D (IgD) is normally present in only low concentrations in serum. In the hyper-IgD and periodic fever syndrome (HIDS), however, serum levels exceed 140 mg/l. This syndrome is further characterized by recurrent inflammatory febrile attacks together with an acute phase response and appearance of cytokines in the circulation. The role of IgD in the pathogenesis of HIDS and its relation to the increased cytokine concentrations is unclear. Therefore, we tested whether IgD, IgG and a 1 -acid glycoprotein (AGP) isolated from human serum influence the synthesis of interleukin-1b (IL-1b), tumour necrosis factor-a (TNF-a), and IL-1ra, as measured by specific radioimmunoassays, in human peripheral blood mononuclear cells (PBMC). Incubation of PBMC with IgD and AGP for 24 hr led to increased release of IL-1b, TNF-a, and IL-1ra. The magnitude of stimulation of IgD exceeded that of AGP; the effect by IgD was dose-dependent and showed a 30-fold (TNF-a) to almost 150-fold (IL-1b) increase at the highest concentration (50 mg/l), while AGP (750 m g/ml) only increased the cytokine secretion fourfold (TNF-a) to almost 30-fold (IL-1b). The effect of IgD on IL-1ra was less dramatic but a fivefold increase was observed at 50 mg/l compared with a 2 . 5-fold increase with AGP. IgD potentiated the effect of lipopolysaccharide (LPS) on secretion of both IL-1b and TNF-a, although the effect was most apparent for TNF-a. Apart from inducing IL-1ra synthesis, IgG did not influence cytokine release in human PBMC. These data indicate that IgD is a potent inducer of TNF-a, IL-1b and IL-1ra and thus may contribute to the pathogenesis of HIDS.