Id reaction or autosensitization dermatitis is an immunological reaction that occurs in response to a distant localized inflammatory/infectious skin condition. It most commonly occurs in response to a local fungal infection occurring elsewhere. The classical example of this reaction is tinea pedis leading to an eczematous eruption on the patient’s hand.


The precise pathophysiology of this condition is not yet fully understood. It is postulated that this reaction happens as a secondary immunological reaction induced by circulating antibodies or activated T cells directed against the microbial agents. Infections that could lead to an Id reaction include fungal, viral, bacterial and arthropod infestations. Furthermore, inflammatory conditions such as chronic venous eczema, acute contact dermatitis, and discoid eczema could also cause an Id reaction.

Clinical presentation: 

Id reaction exhibit a wide variety of presentations, including localized or widespread vesicular lesions, maculopapular eruptions, erythema nodosum, erythema multiforme, erythema annulare centrifugum, Sweet’s syndrome, guttate psoriasis, and autoimmune bullous disease. Although any of these presentations could occur as a reaction to any inflammatory/infectious skin condition, there are some common patterns observed. For example, localized vesicular lesions are usually seen with tinea pedis; erythema nodosum is seen more with bacterial, subcutaneous, and deep fungal infections; and erythema multiforme with herpetic infections. Moreover, regardless of the cause, this reaction most commonly occurs on the medial and lateral aspects of the fingers. Less frequently, this reaction could happen on the chest or arms as well.


Treating the underlying infection (primary source of infection) is usually sufficient to resolve the issue. For instance, treating tinea pedis (athlete’s foot) with a topical antifungal agent should lead to a prompt resolution of the vesicular lesions on the fingers. However, oral or topical steroids may sometimes be needed.

Written by: Rema Aldihan, Medical student.


Ilkit M, Durdu M, Karakaş M. Cutaneous id reactions: a comprehensive review of clinical manifestations, epidemiology, etiology, and management. Critical reviews in microbiology. 2012 Aug 1;38(3):191-202.