As with an exposure to a virus, a contact with an allergen calls on the body's immune system. An allergen is a substance which tends to cause the immune system to react in much the same way as a virus would cause a reaction.
When there is a contact with an allergen without the protection of the skin (1) the allergen goes through the skin and causes an immunological reaction (2). The visible result of this reaction is called eczema. Eczema can appear at any point in a professional career from the first month to many years on.
The skin is sprinkled with pimples, often fills with a clear liquid and is very itchy.