The study investigates the metabolism of human immunoglobulin D (IgD) in 28 patients with various immunoglobulin disorders. IgD, the least abundant immunoglobulin, has a median serum concentration of 30 μg/ml, about 1/40 of IgG. The metabolic behavior of IgD was examined using radioactive iodine-labeled IgD, revealing that the synthetic rate is the primary factor controlling serum IgD concentration. IgD is metabolized independently of other immunoglobulins, with a median synthesis rate of 0.4 mg/kg/day, significantly lower than IgG and IgA. The median serum level of IgD was only 0.2% of the median IgG level, reflecting its rapid catabolism. The biological half-life of IgD was 2.8 days, shorter than IgM (5 days) and IgA (6 days). The fractional catabolic rate of IgD was 0.37, similar to IgA, but significantly higher than IgG. IgD was predominantly found in the intravascular compartment (73%), unlike other 7 S immunoglobulins. The study also found a negative correlation between serum IgD concentration and fractional catabolic rate, which is opposite to the relationship observed for IgG. These findings highlight the unique metabolic characteristics of IgD compared to other immunoglobulins.The study investigates the metabolism of human immunoglobulin D (IgD) in 28 patients with various immunoglobulin disorders. IgD, the least abundant immunoglobulin, has a median serum concentration of 30 μg/ml, about 1/40 of IgG. The metabolic behavior of IgD was examined using radioactive iodine-labeled IgD, revealing that the synthetic rate is the primary factor controlling serum IgD concentration. IgD is metabolized independently of other immunoglobulins, with a median synthesis rate of 0.4 mg/kg/day, significantly lower than IgG and IgA. The median serum level of IgD was only 0.2% of the median IgG level, reflecting its rapid catabolism. The biological half-life of IgD was 2.8 days, shorter than IgM (5 days) and IgA (6 days). The fractional catabolic rate of IgD was 0.37, similar to IgA, but significantly higher than IgG. IgD was predominantly found in the intravascular compartment (73%), unlike other 7 S immunoglobulins. The study also found a negative correlation between serum IgD concentration and fractional catabolic rate, which is opposite to the relationship observed for IgG. These findings highlight the unique metabolic characteristics of IgD compared to other immunoglobulins.