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An "autoimmune" immune response (such as in lupus) can result in the production of abnormal antibodies, called autoantibodies. These autoantibodies are directed against constituents of our own cells, most often located in the nucleus of these cells, hence the term antinuclear (nuclear = coming from the nucleus).
These antinuclear antibodies are directed against compounds in the nucleus of our cells, such as the chromosomes’ DNA (anti-native DNA antibodies), and against other structures called ribonucleoproteins, for example anti-Ro/SS-A, anti-La/SS-B, anti-Sm or anti-RNP. This somewhat complicated terminology designates either the letters of the name of the patient in whom the autoantibody was first described (example: Ro, La, Sm), or the structure recognised by this autoantibody (example: RNP for ribonucleoprotein), or a disease associated with this antibody (example: SS-A for Sjögren Syndrome A).
How are these autoantibodies detected?
These autoantibodies are detected in the patients’ blood by routine laboratory tests. Their detection is done in two stages. First, a screening step affirms the presence and concentration of these autoantibodies. This concentration is expressed by a measurement of serum titration concentrations (1/80, 1/160, 1/320, 1/1280...). This figure means, for example, that there are still visible antibodies in a serum that has been diluted 320 times: this serum is said to be 1/320 positive. The positivity threshold varies between laboratories, but generally in adults, it is considered positive as from a dilution of 1/160.
The second step is to identify the cellular antigen that is targeted by the autoantibody. This step uses specific tests, which will detect antibodies against native DNA, anti-Sm, anti-RNP, anti-Ro/SS-A... They are carried out in many laboratories.
Usefulness of autoantibodies in the diagnosis and prognosis
These autoantibodies serve as diagnostic markers, because in practice the absence of anti-nuclear antibodies rules out the possibility of a systemic lupus. However, they are not specific to lupus because they can be present in many other diseases and even in healthy people. These antibodies can also have prognostic value, because some of them act directly by inducing lesions (example: anti-native DNA antibodies induce kidney damage).
Autoantibodies are antibodies directed against constituents of our own cells. They are detected in lupus and various other systemic autoimmune diseases. These autoantibodies preferentially recognise constituents of cell nuclei, hence the term antinuclear antibodies. In lupus, these antinuclear antibodies are most often anti-native DNA, anti-Ro/SS-A, La/SS-B, RNP and anti-Sm.
The search for antinuclear antibodies is the first biological step towards the diagnosis of lupus.