Regarding  anti-transglutaminase (anti-tTg) IgA, this is one of the tests used to diagnose celiac disease, or gluten sensitivity, an auto-immune disease which results from inappropriate response of the immune system to gluten, the protein found in wheat, barley and rye. This blood tests is performed in conjunction with two other tests:

1- Quantitative immunoglobulin A (IgA) test: ordered along with, prior to, or after an anti-tTG test to check if there is IgA deficiency, which occurs about 2-3% of the time in people with celiac disease and can lead to a false negative result

2- Anti-endomysial antibodies (EMA), which are antibodies developed in reaction to the ongoing damage to the intestinal lining (intestinal mucosa). 

Celiac disease can lead to a number of health damages on the long term, such as unexplained anemia, vitamin deficiency, and others.