Anti-nuclear antibodies (ANA) are autoantibodies produced by the immune system that target proteins within the nucleus of cells. In healthy individuals, the immune system does not attack the body's own tissues. In autoimmune conditions, this self-tolerance breaks down and the immune system generates antibodies against nuclear components including DNA, histones, and nuclear ribonucleoproteins.

ANA testing is performed by indirect immunofluorescence, where the patient's serum is applied to cells on a slide and the pattern and titre of fluorescence are assessed. ANA is a broad screening test — a positive result does not diagnose a specific disease but indicates that autoimmune activity is present. Further specific antibody tests (such as anti-dsDNA, anti-Sm, anti-Ro, anti-La, anti-Scl-70) are then used to identify the precise autoimmune condition.