Classification of inflammatory skin diseases: A proposal based on the disorders of the three-layered defense systems, barrier, innate immunity and acquired immunity

• Inflammation is defined as a series of protective and regenerative responses of the body.
• Host defense system of the skin is composed of a barrier, innate immunity and acquired immunity.
• These three layers of the defense system drive the most suitable response against external dangers.
• Disorder of the specific layer of the defense system can induce an inflammatory skin disease.
• Hence, dermatitis partly simulates the actual protective response against infections and dangers.

The host defense system of the skin is composed of (1) a barrier, (2) innate immunity, and (3) acquired immunity. Inflammatory skin diseases can be classified into one of the disorders of these layers of the defense system, unless there is an ordinary response to specific infectious agents or internal/external injury. Any inflammatory skin disease partly simulates the response to real infections or dangers. Disorders of acquired immunity can be classified into (1) immunodeficiency, (2) immunohyperactivity (allergy), and (3) qualitative disorder (autoimmunity). Disorders of innate immunity can be classified into (1) innate immunodeficiency, (2) innate immunohyperactivity (general or local autoinflammation), and (3) qualitative disorder (general or local innate autoimmunity). The barrier of the skin is composed of (1) the physical barrier and (2) the chemical barrier. Several diseases, such as atopic dermatitis, are attributed to the disorder of these components of the barrier. Here, we propose an algorithm to classify the pathology of inflammatory skin diseases by means of what disorder in the specific layer of the host defense system is truly responsible.