A number of diseases are due to the immune system failing to recognise the body’s antigens as "self" and mounting an immune response to host antigens. These "auto-immune diseases" may manifest themselves by the presence of auto-antibodies and detection of these can be a valuable aid to diagnosis. If an auto-immune disease is suspected then a request for an "auto-immune screen" is appropriate. This screen comprises:

Anti-Nuclear Antibodies. (ANA)
High levels of ANA are associated with connective tissue disease such as systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). An immunofluorescence technique is used to analyse ANA and the pattern obtained may give a clue to the target antigen and therefore the disease process involved (e.g a homogenous pattern is associated with SLE). If further analysis is required then appropriate advice will be given.

Anti-Mitochondrial Antibodies. (AMA)
These are found in high titre in primary biliary cirrhosis (PBC) and also in lower levels in auto-immune hepatitis. (AIH). The two conditions are associated with different target antigens and further tests are available to distinguish between the two (see later – liver profile).

Anti-Smooth Muscle Antibodies. (SMA)
In high titre these are associated with auto-immune hepatitis but may also be found in low titre in PBC and infection.

Anti-Gastric Parietal Cell Antibodies. (GPCA)
These are found in ninety five percent of people with pernicious anaemia (PA). A positive result should be followed by an Intrinsic Factor Antibody (IFA) test which, when positive, confirms a diagnosis of PA.

More specific auto-antibody tests :

Double stranded DNA antibodies. (dsDNAabs)
This test should be requested where the ANA is positive and SLE is suspected. A positive result is associated with SLE.

Extractable Nuclear Antigen. (ENA)
These include antigens designated RNP, Sm, Ro (or SS-A), La (or SS-B), Scl-70, Jo-1 and histone each of which corresponds to a specific nuclear antigen. Antibodies to these specific antigens are associated with particular disease conditions (eg Ro and La with SLE or Sjogren’s syndrome and Jo-1 with polymyositis).

Centromere antibodies.
Anti centromere antibodies are associated with a variant form of scleroderma termed the CREST (calcinosis, Raynaud’s, oesophageal dysmotility, sclerodactyly, telangiectasia) syndrome.

Liver Profile
This is a compound test involving three antigens; M2, LKM and SLA/LP. Antibodies to M2 are strongly associated with PBC whilst antibodies to LKM and SLA/LP are associated with auto-immune hepatitis.

Rheumatoid Factor (RF)
RF is an antibody which targets the tail portion (Fc region) of IgG with a resulting inflammatory response. It is found in a number of auto-immune and inflammatory diseases and also in healthy individuals, the frequency increasing with age. In rheumatoid arthritis the RF may be positive or negative although patients with severe disease tend to have higher levels. In such cases there is no value in serial monitoring of RF. CRP is a better indicator of inflammation.

Anti Neutrophil Cytoplasmic antibody (ANCA)
These antibodies react with cytoplasmic constituents of neutrophils. There are two major patterns of clinical significance. C-ANCA, where the antigen is proteinase-3 (Pr 3), is strongly associated with Wegener’s Granulomatosis. P-ANCA, where the antigen is myeloperoxidase, is less specific and may be positive in a number of vasculitic conditions.

Glomerular Basement Membrane (GBM) antibodies
Anti GBM antibodies are associated with the glomerulonephritis of Goodpasture’s syndrome. They target a collagen constituent of the glomerular basement membrane and the resulting inflammation results in glomerular damage.

Sperm antibodies
These are found in males and females and may be implicated in infertility. They are also found in males after vasectomy.