ID REACTION

An id reaction (from the Greek for Father-Son relationship), also known as autosensitization is a secondary dermatitis. It is a systemic, allergic, dermatitis to a remote antigen. The dermatophytid is the most frequently referenced id reaction in dermatology. 

Etiology:  
A dermatophytid is an autosensitization reaction where a secondary cutaneous reaction occurs at a site distant to a primary fungal infection. The condition has no known predilection for any racial group or sex. Apart from infections with dermatophytes, other causes of id reaction include infection with mycobacteria, viruses, bacteria, or parasites (pediculosis), contact dermatitis, stasis dermatitis, or other eczematous dermatoses.

Clinical features:  
An id reaction is an intense inflammatory disease of the skin. Characterized by itchy, vesicular rash that usually occurs on the chest, arms, or sides of the fingers. This rash occurs in response to an intense inflammatory process somewhere else on the body. Treating the fungal infection makes the rash on the finger go away. Sometimes an id reaction is the only clinical manifestation of athlete's foot. Careful inspection of the toe web spaces of a person with an id reaction often reveals an asymptomatic fissure or maceration. The lesion can have many appearances. The classic is an itchy, dyshydrotic-like vesicular lesion on the fingers. The more systemic form can look like small vesicles or a diffuse blanching eythroderm. Secondary bacterial infection can occur.

Pathogenesis:  
The distant eruption is due to circulating fungal antigen from the primary site reacting with antibodies at sensitized areas of the skin. It is probably, at least in part, IgE mediated although the exact pathophysiology is not well understood. While the exact cause of the id reaction is unknown, the following factors are thought to be responsible: (1) abnormal immune recognition of autologous skin antigens, (2) increased stimulation of normal T cells by altered skin constituents, (3) dissemination of infectious antigen with a secondary response, and (4) hematogenous dissemination of cytokines from a primary site.

Diagnosis and treatment:  
No fungal element can be demonstrated at the site of id reaction. Skin tests for trichophytin antigen are usually positive. Treatment of the underlying fungal infection may relieve the patient of id reaction. Symptomatic treatment of pruritus with antihistamines and/or topical steroids may be undertaken.