Background: The function and regulation of circulating IgD are unclear. Serum IgD levels were increased in a wide range of immunological diseases but these associations did not give a clue to the regulation of serum IgD production. Methods: Serum IgD levels in 131 children with various non-HIV-related immunodeficiency diseases were investigated to examine their relationship with immunoglobulin or antibody production and activation of the immune system. Data from a group of 109 nonimmunodefícient children were also available for comparison. Results: There was a bimodal distribution of serum IgD levels. In 87 patients IgD levels fell below the limit of detection of 5 IU/ml, while the remainder showed an approximately normal distribution skewed to the right after log transformation. The proportion of children with undetectable IgD levels (Conclusion: Determination of serum IgD levels did not seem to be of particular clinical benefit in the investigation of HIV-negative immunodeficient children and serum IgD levels were not associated with the general picture of immune activation. Observed distribution patterns and associations may have implications for the regulation of serum IgD production
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