Dogs with atopic dermatitis have a defective skin barrier. Allergens such as pollens and dust mite allergens actually cross the skin barrier and trigger an immune response by langerhans cells. This sets up an allergic reaction which triggers histamine release and an itchy rash develops.
That is a very simplified account of the immunology. The take home point is that the allergens pass transdermal, ie across the skin barrier.
In a normal dog, the epithelial cells are held together by a ceramide layer. This is much like a mortar holding bricks together in a brick wall. It's easy to imagine the external skin as a brick wall. In dogs with atopic dermatitis, they are missing a protein called sphingosine in the ceramide layer.
The skin is therefore 'leaky'.
A feature of dogs with atopic dermatitis is that they have very dry skin. these dogs have very dry flakey skin due to moisture loss. Water escapes through the skin due to the faulty ceramide layer. Moisture loss leads to very dry skin.
The other problem with this leaky skin is that allergens such as pollen can penetrate the outer layers of the skin. Bacetria can also colonise the deeper layers of the skin and they secrete enzymes which further damage the ceramide layer making it even more leaky.
Dogs with atopic dermatitis have very dry skin. They typically get a rash from an allergic reaction on the skin which has no hair covering such as the feet, the abdomen, arm pit and groin and also the concave surface of the ear.
Recent therapy involves repairing the defective lipid barrier and protecting the dog from moisture loss through the skin.
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