Anti-Smith (Sm)

The Sm antigen is a complex of nuclear non-histone proteins that was characterized in 1966 and was the first nuclear protein autoantigen to be described in SLE. The antigen to which anti-Sm antibodies bind consists of a series of proteins: B; B'; D1; D2: D3; E; F; and G, complexed with small nuclear RNAs: U1; U2; U4-6; and U5. These complexes of nuclear proteins and RNAs are called small nuclear ribonucleoprotein particles (snRNPs); they are important in the splicing of precursor messenger RNA.
Antibodies to Sm antigen are found in an average of 5-30% of patients with SLE, and are considered a marker antibody besides anti-dsDNA2-5. Sm antibodies have been included in the American College of Rheumatology criteria for the classification of SLE. (Source: Package insert: QUANTA Flash Sm Reagent. Inova Diagnostics. 03/2025).