What they did
The authors review the evolution of our current understanding of the pathophysiology of atopic dermatitis (AD) and the role allergens and the immune system play in its manifestation. Recent research has redefined the role of IgE and histamine in the pathogenesis of this disease.

What they found
Historically, AD was described as a type 1 hypersensitivity reaction in the skin mediated by allergen-specific IgE antibodies. Reexposure to the allergen would trigger mast cell degranulation and histamine release, leading to pruritus and inflammation.

It is now recognized that AD can occur even in the absence of allergen-specific IgE and that, conversely, allergen-specific IgE concentrations can be detected in dogs that do not have clinical manifestations of AD. Other factors such as T cell subpopulations, altered mast cell releasability, and defective skin barrier function may also contribute to the pathogenesis of AD.

Histamine as a mediator of pruritus in canine AD has also been challenged in recent studies. Cytokines and leukotrienes are being actively studied as contributors to the inflammatory response in patients with AD. Ultrastructural changes in the skin surface have also been documented in dogs with AD, which may predispose percutaneous allergen exposure. Whether AD is caused by a primary immune defect or the result of a defect in the skin barrier is still a matter of debate according to the review authors.

Take-home message
Despite greater insight into the pathogenesis of AD, complex immune system interactions make definitive diagnosis and treatment challenging. The authors note that exclusion of other diseases such as infections, ectoparasitic infestations, and food allergies will still be required before intradermal and serologic tests are performed. "Serum-based diagnostic or genetic testing that could be used in collaboration with clinical assessments may help veterinarians prescribe the most appropriate treatments for AD patients more quickly," note the authors.

The authors also state that with this new information, it will eventually be possible to develop safer and more effective treatments that target AD at many points in the cycle.

Marsella R, Sousa CA, Gonzales AJ, et al. Current understanding of the pathophysiologic mechanisms of canine atopic dermatitis. J Am Vet Med Assoc 2012;241(2):194-203.