Pathogenesis
Potential explanations include

Atypical immune recognition of autologous skin antigens
Stimulation of normal T cells by changing skin constituents
Lower threshold for skin irritation
Spreading of infectious antigens causing a secondary response
Hematogenous dissemination of cytokines from the primary site of inflammation

Diagnosis
Although there are a multitude of varying appearances, the id reaction often presents with symmetrical red patches of eczema with papules and vesicles, particularly on the outer sides of the arms, face and trunk which occur suddenly and are intensely itchy occur a few days to a week after the initial allergic or irritant dermatitis. Most commonly, athletes foot can lead to localised vesicles on hands, bacterial infections to erythema nodosum and herpes simplex virus to erythema multiforme.The diagnosis is frequently made by treating the initial triggering skin problem and observing the improvement in the eczematous rash. Both the initial skin problem and the id reaction must be observed to make the diagnosis.Not all dyshidrotic rashes are 
id reactions, but id reactions are often dyshidrotic-like.Initial tests may include isolating a fungus by taking a swab and sending it for culture. Patch testing may be considered if there is suspicion of allergic contact dermatitis.A skin biopsy is rarely necessary, but if done mostly shows an interstitial granulomatous dermatitis, some lesions being spongiotic. Id reactions cannot be distinguished from other skin diseases by histopathology. However, they can be distinguished from other id reactions by histopathology. Differential diagnosis
Other rashes that occur in a widespread distribution can look like an id reaction. These include atopic dermatitis, contact dermatitis, dyshidrosis, photodermatitis, scabies and drug eruptions.
Contrary to pleomorphic adenoma, it is non recurrent. See also
Pleomorphic adenoma

References
Additional sources
Kahn, Michael A. Basic Oral and Maxillofacial Pathology. Volume 1. 2001. == External links ==