In contact eczema the dermal appendages afford a mode of entry for the allergen into the epidermis and dermis, causing changes in both these structures, destructive in the epidermis and exudative in the dermis. Oedema fluid and lymphocytes from the upper dermis then force their way into the weakened epidermis to form vesicles. The degree of the reaction varies considerably in intensity from case to case. Secondary sensitisation eruptions also have their main impact on and around the dermal appendages, as a result of which intercellular oedema and vesiculation occur in the epidermis in relation to those appendages. At a later stage in the secondary reaction, hypertrophy of the sweat ducts takes place, and these hypertrophied ducts adhere to one another in the upper dermis with resultant acanthosis.