There are two classes of antibodies seen in untreated celiac disease. Antibodies directed against a fragment of gluten called gliadin, and antibodies directed against a particular tissue in the body itself. The two main areas in the body which can be attacked by its own antibodies are the aendomysial (the covering of muscle), and the reticulin ( the framework for kidney and liver), but there are others.
How good are these tests?
If all of the blood test results are positive a celiac disease diagnosis is 90% accurate. However, there are several circumstances in which the tests can be inaccurate. IGA and IGG are two different varieties of antibodies which are produced by most peoples immune systems. There is a different blood test for each of the antibodies. Of the two tests, the IGA gliadin and IGA endomysial tests are the most accurate. However, this test can become negative relatively quickly after going on a gluten-free diet (3-6 months), which can cause a false negative test result. The IGG is less specific, and can sometimes be positive in non-celiacs. Also, about 4% of celiacs have no IgA at all! For these reasons it is very important that both tests are done for an accurate diagnosis. The biopsy is still considered the "standard candle" to confirm a blood diagnosis, and give a 100% sure diagnosis.
For all tests for celiac disease it is necessary that one is on a gluten-containing diet, or false-negative test results could be given. Blood tests may also be useful in following up a known celiac and confirm that the diet is indeed free of large amounts of gluten. Also, because of the lack of standardization, keep in mind that blood test results may not be directly comparable from one lab to the next.