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Endogenous factors
Atopy
Dry skin
Dyshidrosis
Hyperhidrosis
Abnormal sebum composition and commensal Malassezia furfur
Exogenous factors
Contact allergens
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Phototoxic agents


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Exogenous factors - Contact allergens

Eczema may also be caused by environmental factors. These exogenous factors include contact allergens, irritants, photoallergens and phototoxic agents.

Contact allergy results from a specific acquired hypersensitivity of the delayed type, also known as cell-mediated hypersensitivity (type IV allergy according to Coombs and Gell). For sensitisation individuals must have significant and normally repeated contact with the allergen. Therefore individuals may be exposed to allergens for years before finally developing hypersensitivity. Numerous allergens have been identified as causes for allergic contact dermatitis. The most common contact allergens are nickel salts and fragrance mix (Table 1).

The pathway of sensitisation:
Most allergens are of small molecular weight, so-called haptens. After penetrating the skin haptens form a complete antigen by binding to a structural or cell surface protein. The antigens are processed by Langerhans cells (or other antigen presenting cells) and presented to T lymphocytes. Keratinocytes influence this induction phase of sensitisation releasing cytokines. The T lymphocytes migrate to the regional lymph nodes where there is an activation of naive T cells resulting in the generation of T effector cells.
Later contact with the allergen in a sensitised individual causes the activation of T effector cells by antigen presentation either by resident cells or by infiltrating Langerhans cells. Subsequently other inflammatory cells including granulocytes and macrophages are attracted which ultimately cause the clinical manifestation of contact dermatitis. The reaction is mainly coordinated by cytokin release and expression of endothelial adhesion molecules which attracts leukocytes to the site of antigen contact.

Commonly eczema caused by exogenous agents occurs at the site of contact. In disseminated eczema additional patches arise at parts of the skin which had not been in direct contact with the eliciting agent. Its distribution reflects systemic (hematogenous) dissemination of allergens. However, the pathogenesis is not really understood.

Table 1:
Important contact allergens in Germany in the year 2000 (IVDK = Informationsverbund Dermatologischer Kliniken):

Allergens

Positive reactions (%)

Appearance

Nickel sulfate

17.6

jewellery, cutlery, zipper, wristwatch

Fragrance mix

10.2

cosmetics, perfume, cleaning agents

Balsam of Peru

8.4

scent of cosmetics, topical medicaments, tobacco

Thimerosal

7.3

preservative in vaccines, eye drops

Cobalt

5.3

jewellery, cutlery, zipper, wristwatch, cement, hair tinting lotion

Lanolin

4.3

creams, ointments, printing ink,

Colophony

4.2

paper, band-aid, adhesives, polish, cosmetics

Para-phenylendiamine

4.0

hair tinting lotion, paints

MDBGN/PE

3.8

preservative in cosmetics, pharmaceuticals

Potassium dichromate

3.4

cement, leather, impregnation, galvanisation

Thiuram mix

2.4

any kind of vulcanised rubber

Formaldehyde

1.8

desinfectant, plastics

MDBGN/PE: methyldibromo-glutaronitrile/phenoxyethanol (Euxyl K400)



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