Leg affected by dermatitis herpetiformis. This is a chronic skin disease in which tiny, red and itchy blisters occur in a symmetrical pattern on various parts of the body. It affects most commonly the back, elbows, knees, buttocks and scalp. It is believed that dermatitis herpetiformis is related to coeliac disease, a condition in which the small intestine is allergic to gluten, a constituent of wheat and other cereals. Both conditions often improve by following a gluten-free diet.