I have a barley allergy confirmed with a blood test. Was tested for wheat, rye. Is it common for barley allergy to "standalone" or similar substances?

Barley allergy. Yes barley allergy alone is actually very common. I usually see 6-7 people a year with this allergy. Usually it causes itching alone (which can lead to eczema), recurrent urticaria ( hives) and/or intestinal cramping with diarrhea. The hard part with avoidance is that many brands of flour are a wheat/ barley mix. So this calls for a lot of label reading and avoidance of things made with flour .