There are patients with "gluten sensitivity" in whom neither allergic nor autoimmune mechanisms can be identified

From BMJ:

It is now becoming clear that, besides those with coeliac disease or wheat allergy, there are patients with gluten sensitivity in whom neither allergic nor autoimmune mechanisms can be identified. It has been estimated that, for every person with coeliac disease, there should be at least six or seven people with non-coeliac gluten sensitivity. Gluten sensitivity may therefore affect 6-10% of the general population.

Patients with gluten sensitivity have negative immuno-allergy tests to wheat and negative coeliac disease serology; normal endoscopy and biopsy; clinical symptoms that can overlap with those of coeliac disease, irritable bowel syndrome, and wheat allergy; and they show a resolution of symptoms when started on a gluten-free diet.

Currently there are no laboratory biomarkers specific for gluten sensitivity, and the diagnosis is based on exclusion criteria; elimination of gluten-containing foods from the diet followed by an open challenge is most often used to establish whether health improves with the elimination or reduction of gluten from the patient’s diet.

References:

Non-coeliac gluten sensitivity | BMJ http://bit.ly/SlOTNO
Celiac disease and nonceliac gluten sensitivity - 2017 review in Annals of Allergy, Asthma & Immunology http://buff.ly/2pI2x61