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There are two major reasons for eczema - Impaired epithelial barrier function of the skin (due intrinsic structure or functional problem) . Second is an immune function disorder.
The epidermis is the first line of defense between the body and the environment. An intact epidermis keeps environmental irritants, allergens, and microbes from entering the body . Any disruption of these components through inherited defects, trauma, decreased humidity, alteration of pH, and infection can interfere with the ability of the epidermis to function as an effective barrier. Disruption allows antigenic and irritant agents to penetrate the barrier and come into contact with immune cells, leading to the release of proinflammatory mediators . This can then produce the clinical and pathologic findings of dermatitis - or eczema.
Immune dysfunction is another cause. This is more complex and technical but basically the immunity which is supposed to fight of infections and inflammations either over act / respond poorly. This can result in eczema. Filaggrin mutation Spink 5 dysfunction, lack of epithelial hydration and some microbial infections can also cause eczema.