Antibodies against the Sm antigens of the small nuclear ribonucleoproteins (snRNPs) are a very specific marker for the diagnosis of systemic lupus erythematosus (SLE). Antibodies against the Sm proteins are found in 20-30% of SLE patients. Anti-Sm antibodies are observed at a high titer in the active period of SLE and at a low titer in the non-active period. Antibodies directed against Sm proteins as well as DNA found in sera from patients with SLE have been included as one of the American College of Rheumatology classification criteria for SLE. Sm antigen is now known to be consisted of at least nine different polypeptides. Anti-Sm antibodies are predominantly directed against the SmD1 protein of the snRNP complex.
Autoimmune ELISA kits measure autoimmune antibodies in the serum. It is based on the principle of a solid phase enzyme-linked immunosorbent assay. The assay utilizes a specific antigen for immobilization on the microtiter wells and anti-human IgG antibody conjugated to horseradish peroxidase (HRP) for detection. The test sample is allowed to react simultaneously with the two components, resulting in autoimmune antibodies being sandwiched between the solid phase antigen and the enzyme-linked antibodies. After incubation, the wells are washed to remove unbound antibodies. Then an HRP substrate is added to develop a blue color. The color development is stopped with the addition of Stop Solution changing the color to yellow. The concentration of autoimmune antibodies is directly proportional to the color intensity of the test sample. Absorbance is measured spectrophotometrically at 450 nm.