Id Reaction



A generalized skin reaction associated with various infectious (fungal, bacterial, viral, or parasitic) or inflammatory cutaneous conditions distant from the primary disease site (1)

  • “Id” is often combined with a root to reflect the causative factor (i.e., bacterid, syphilid, and tuberculid). Dermatophytid is the most frequently referenced id reaction. A dermatophytid is an autosensitization reaction in which a secondary cutaneous reaction occurs at a site distant to a primary fungal infection. The eruption typically begins within 1 to 2 weeks of the onset of the main lesion or following exacerbation of the main lesion.
  • Most commonly localized vesicular lesions, erythema nodosum, and erythema multiforme; more uncommonly, can cause presence of vesicles and pustules
  • System(s) affected: skin/exocrine
  • Synonym(s): dermatophytid, trichophytid, autoeczematization


  • Predominant age: all ages
  • Predominant sex: male = female
  • Predominant race: all races



Etiology and Pathophysiology

Precise pathophysiology is uncertain. Circulating antigens may react with antibodies at sensitized areas of the skin. An abnormal immune recognition of autologous skin antigens may also occur. Inflammation may lower the irritation threshold of the skin, and hematogenous spread of cytokines from the primary site of inflammation may also play a role (1).

  • Etiology
    • Infectious
      • Fungal infections: Trichophyton mentagrophytes, Trichophyton rubrum, Epidermophyton floccosum, and Candida spp.
      • Bacterial infections: Streptococcus pyogenes, Staphylococcus aureus, and Mycobacterium tuberculosis
      • Viral infections: HSV, Molluscum contagiosum, orf, and milker’s nodules
      • Parasitic infections: Sarcoptes scabiei, Leishmania spp., and Pediculus humanus capitis
    • Allergic
      • Id reactions occur in patients with nickel and aluminum allergy or, less commonly, second to concurrent treatment with antibacterial agent and terbinafine.
    • Miscellaneous
      • Id reaction rarely develops due to retained postoperative sutures, cyanoacrylate application, ionizing radiation, blunt trauma, red tattoo ink, postpicosecond laser tattoo removal.
      • Rarely, id reaction has been documented in patients receiving intravesical BCG live therapy for transitional cell carcinoma.

Risk Factors

  • Fungal infection of the skin, especially tinea pedis
  • Stasis dermatitis

General Prevention

  • Good skin hygiene (particularly in intertriginous areas) to minimize risk of developing fungal infections
  • Promptly treat any developing fungal infection.

Commonly Associated Conditions

  • Primary fungal infection
  • Stasis dermatitis

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