"The rash is caused when gluten in the diet combines with IgA, and together they enter the blood stream and circulate."Interestingly, iodine is required for the reaction so avoiding iodized salt is recommended. If you have this kind of rash, you should see a doctor who understands celiac disease. More than 90% of patients with Dermatitis Herpetiformis also have gluten sensitive enteropathy which is celiac disease. I would also recommend getting a copy the book Celiac Disease: A Hidden Epidemic by Dr. Peter Green.
If you have an undiagnosed, untreated autoimmune disease then you are more likely to get a second autoimmune disease so getting a diagnosis and treatment if you have this type of rash is important. The good news about having this itchy skin condition is that it goes away on a gluten free diet.