Clinical immunology
Antibodies reacting with human immunoglobulin in sera from autoimmune thyroid disease patients as a risk factor for false positive results in IgA assessment

Autoimmune thyroid diseases (ATD) are organ-specific autoimmune disorders namely, Hashimoto’s thyroiditis and Graves’ disease, characterized by the production of antibodies against the thyroid peroxidase and thyroid-stimulating hormone receptor in the thyroid gland. The purpose of this study was to optimize the ELISA tests for quantifying serum immunoglobulins in ATD patients by minimizing the interference of anti-goat immunoglobulin and to evaluate the prevalence effect on the measurement of the humoral immune response. Anti-immunoglobulin antibodies were defined using goat immunoglobulins as a target to characterize distinct changes in patterns of immunoglobulin levels in ATD patients. Concentrations of serum immunoglobulin A, G and M in thirty five patients with ATD with positive anti-thyroid antibodies and thirty five matched normal healthy adult individuals were tested by ELISA. The effect of anti-goat immunoglobulin antibodies leads to an overestimation of serum IgA level in ATD and is different for each serum sample tested. Initial results obtained before purification from the interfering anti-goat immunoglobulin antibodies suggested that ATD patients had increased levels of IgA in their sera. It was found that normal individuals had mean IgA, IgG and IgM levels of 2.75 mg/ml, 9.67 mg/ml and 1.81 mg/ml, respectively while individuals suffering from ATD had mean levels of 5.01 mg/ml, 10.17 mg/ml and 1.92 mg/ml (p<0.0004, p<0.20 and p<0.34). However, the mean level of IgA in ATD sera treated from anti-goat immunoglobulins was determined to be 2.81 mg/ml. Therefore, there was no significant difference in IgA level in patients with ATD compared to normal individuals (p<0.63) after removal of anti-goat immunoglobulin antibodies. Visualization of IgA by immunoblotting confirms that anti-goat immunoglobulins antibodies, which were unrelated to antigen, were co-precipitated with the antigen-antibody complex. A circulating immunoglobulin reacting with other immunoglobulins is thus present at increased levels in adult subjects with ATD and may well play a part in the complex immunopathogenetic interactions.