ANA: Antinuclear antibody, an unusual antibody directed against structures within the nucleus of the cell. ANAs are found in patients whose immune system is predisposed to cause inflammation against their own body tissues. Antibodies that are directed against one's own tissues are referred to as autoantibodies. The propensity for the immune system to work against its own body is referred to as autoimmunity. ANAs indicate the possible presence of autoimmunity.