Chapter 3. Dry Skin and Hyperreactivity
The reason that patients with atopic dermatitis are so sensitive to their environment is complex and not completely understood. One important factor is that they have a defect in their skin barrier function. Certain lipids, such as ceramides, are lacking in the stratum corneum or top layer of their skin. This results in water loss from the skin. Dry skin is more brittle and prone to cracking thus creating portals of entry for irritants, allergens and microbes into the deeper layers of skin.
This increased absorption of environmental triggers sets off the skin immune response to protect itself from foreign invasion. A by-product of this protective immune response is the triggering of an intense inflammatory response that leads to the release of chemicals and skin cell activation which cause itching and redness. In a normal individual, this response is short-lived once the foreign material is eliminated. However, in allergic skin there is a greater immediate inflammatory response due to higher levels of local antigen absorption and the skin inflammation is more persistent due to derangements in the immune response of patients with atopic dermatitis.
The term allergen refers to environmental proteins (e.g. foods and inhalant allergens such as dust mites) which trigger allergic reactions in the skin of patients with atopic dermatitis or the respiratory tract of patients with asthma or hay fever. These same allergens usually have no effect on normal individuals. What distinguishes allergic from non-allergic individuals is that allergic patients produce IgE responses against allergens. Once cells in the skin, such as mast cells or antigen-presenting cells, are armed with IgE they are poised to produce immediate and sustained cellular immune responses when they encounter small quantities of environmental allergens. It is estimated that nearly 80% of patients with atopic dermatitis make increased amounts of IgE to environmental allergens. These individuals are referred to as "extrinsic" atopic dermatitis.
The triggers of eczema in the remaining 20% of patients referred to as "intrinsic" or "pure" form of atopic dermatitis are less well understood. It may be that they are also triggered by similar environmental allergens but do not use IgE as a mechanism for activating the immune response. In any case, these patients can be triggered by irritants and microbes in just the same manner as patients with extrinsic atopic dermatitis are. Dry skin and sweating due to excessive evaporation of water from the skin are common triggers for both forms of atopic dermatitis. Climatic changes in temperature or humidity can also trigger eczema. Therefore, skin moisturization is critical for effective management of this skin disease.