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IgG anti-IgA antibodies were measured by an enzyme-linked immunosorbent assay using one IgA1 and one IgA2 (ml) myeloma and a pooled IgA protein preparation as antigens. Class-specific anti-IgA antibodies occurred in 0.8% of non-IgA-deficient sera and 24.3% of IgA-deficient sera. Antibodies reacting with IgA1 only occurred in 2.6% of non-IgA-deficient sera and 6.7% of IgA-deficient sera. Antibodies reacting with IgA2 only occurred in 0.6% of non-IgA-deficient sera and 2.7% of IgA-deficient sera. The prevalence of anti-IgA in IgA deficients with inflammatory disease was higher (81.8%) than in IgA deficients without disease (24.1 %) and was accounted for by class-specific antibodies.

We describe a young girl who presented with musculoskeletal symptoms and who was found to have high titres of antinuclear antibody with anti-RNP antibody. She was initially suspected of having mixed connective tissue disease, but ultimately was found to have metastatic undifferentiated carcinoma with an unknown primary site. This is a very uncommon malignancy of childhood and an association with anti-RNP antibody has, to our knowledge, not been described. The clinical significance of this finding is discussed.

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