# Contact Dermatitis: Allergic and Irritant Mechanisms

## Overview

Contact dermatitis encompasses inflammatory skin reactions resulting from direct exposure to exogenous substances. It is divided into allergic contact dermatitis (ACD), a type IV delayed hypersensitivity reaction, and irritant contact dermatitis (ICD), which results from direct cytotoxic or inflammatory effects without immune sensitization. Contact dermatitis accounts for significant occupational morbidity and is a cornerstone of dermatologic diagnosis.

## Epidemiology

Contact dermatitis accounts for approximately 90 percent of occupational skin diseases. ICD is more common than ACD, representing roughly 80 percent of cases versus 20 percent. The lifetime prevalence of ACD is approximately 15 to 20 percent. The most common allergens vary by geography, but nickel is the most prevalent worldwide. Hand dermatitis is the most common clinical presentation in occupational settings.

## Irritant Contact Dermatitis (ICD)

### Pathophysiology

ICD involves direct damage to the skin barrier without prior sensitization — there is no immunologic memory, and it can occur on first exposure. The mechanisms include disruption of the lipid barrier by solvents and detergents, protein denaturation by acids and alkalis, direct cytotoxicity to keratinocytes, release of pro-inflammatory cytokines (IL-1alpha, TNF-alpha, IL-8) from damaged keratinocytes, and innate immune activation without adaptive immune involvement.

### Clinical Features

Acute ICD presents with erythema, edema, vesiculation, and erosions at the contact site. Chronic ICD shows dryness, fissuring, lichenification, and hyperkeratosis. Margins are typically sharp, conforming to the area of contact. A key clinical distinction from ACD is that ICD symptoms tend toward burning and stinging more than pruritus, and the reaction follows a "decrescendo" pattern — most intense at the center of the contact area.

### Common Irritants

Water (wet work) is the most common occupational irritant. Others include soaps and detergents (sodium lauryl sulfate), solvents (acetone, toluene, alcohol), acids and alkalis, fiberglass particles, irritant plants (euphorbias, poinsettia), and in infants, diaper dermatitis from urine, feces, friction, and occlusion.

## Allergic Contact Dermatitis (ACD)

### Immunologic Mechanism (Type IV Hypersensitivity)

ACD involves two phases. In the **sensitization phase** (afferent limb), a hapten — a small molecule under 500 daltons — penetrates the skin barrier and binds to a carrier protein to form a complete antigen. Langerhans cells and dermal dendritic cells internalize and process this antigen, then migrate to regional lymph nodes via afferent lymphatics. There they present the antigen to naive T cells on MHC class II (and class I for some haptens), driving clonal expansion of hapten-specific effector and memory T cells. Sensitization takes 10 to 14 days on first exposure.

In the **elicitation phase** (efferent limb), re-exposure to the hapten in a sensitized individual leads to local antigen presentation by Langerhans cells and dermal DCs, recruitment of memory T cells (Th1 and CD8+ cytotoxic T cells) to the skin within 24 to 72 hours, and release of IFN-gamma, TNF-alpha, IL-17, and cytotoxic mediators. The result is spongiotic eczematous dermatitis, with peak reaction at 48 to 72 hours — the basis of the patch test reading schedule.

### Clinical Features

Acute ACD shows pruritic erythema, edema, vesicles, bullae, and weeping. Subacute disease shows erythema, papules, scale, and crusting. Chronic disease produces lichenification, fissuring, and pigmentary changes. Distribution often provides the key diagnostic clue: linear or geometric patterns suggest an external contactant; hands suggest occupational exposure; eyelids may indicate cosmetics, nail polish (via ectopic transfer), or airborne allergens; dorsal feet suggest shoe components (rubber, adhesives, chromate in leather); periumbilical distribution suggests nickel from belt buckles or jeans buttons; wrist and earlobe involvement suggests nickel jewelry; and face and neck involvement suggests fragrances or preservatives in cosmetics.

### Top Allergens

| Allergen | Common Sources | Cross-Reactions / Notes |
|---|---|---|
| Nickel sulfate | Jewelry, belt buckles, electronics | Most common worldwide allergen |
| Fragrance mix I and II | Perfumes, cosmetics, household products | Broad cross-reactivity |
| Balsam of Peru | Fragrances, flavorings, spices | Cross-reacts with fragrances and flavorings |
| Neomycin | Topical antibiotics | Cross-reacts with bacitracin |
| Formaldehyde / releasers | Cosmetics, fabric finishes, adhesives | DMDM hydantoin, quaternium-15, imidazolidinyl urea |
| Methylisothiazolinone (MI) | Cosmetics, wet wipes, industrial products | Epidemic of sensitization in the 2010s |
| Cobalt chloride | Metal alloys, paints | Often co-sensitized with nickel |
| p-Phenylenediamine (PPD) | Permanent hair dyes | Cross-reacts with sulfonamides, procaine, PABA |
| Rubber accelerators | Gloves, elastic | Thiuram mix, carba mix, mercaptobenzothiazole |
| Topical corticosteroids | Therapeutic agents | Tixocortol pivalate (group A), budesonide (group B) |

The most common allergens identified by the North American Contact Dermatitis Group include nickel sulfate (jewelry, belt buckles, electronics); fragrance mix I and II (perfumes, cosmetics, household products); balsam of Peru (cross-reacts with fragrances, flavorings, and spices); neomycin (which cross-reacts with bacitracin); formaldehyde and formaldehyde releasers (DMDM hydantoin, quaternium-15, imidazolidinyl urea, diazolidinyl urea) found in cosmetics, fabric finishes, and adhesives; methylisothiazolinone (MI), a preservative that saw an epidemic of sensitization in the 2010s; cobalt chloride (often co-sensitized with nickel); p-phenylenediamine (PPD) in permanent hair dyes (which cross-reacts with sulfonamides, procaine, and PABA sunscreens); rubber accelerators (thiuram mix, carba mix, mercaptobenzothiazole) in gloves and elastic; and paradoxically, topical corticosteroids themselves — tixocortol pivalate screens for group A allergy and budesonide for group B.

## Patch Testing

### Indications

Patch testing is indicated for chronic or recurrent eczema of unknown etiology, eczema in distributions suspicious for ACD, occupational dermatitis, dermatitis unresponsive to appropriate therapy, and eyelid, hand, or foot dermatitis.

### Methodology

The gold standard uses Finn chambers or the T.R.U.E. TEST system. A standard series of approximately 80 allergens (or the local equivalent) is applied to the upper back, occluded for 48 hours. Supplemental series are added based on clinical suspicion. The reading schedule includes an initial reading at day 2 (48 hours) after patch removal, and a critically important delayed reading at day 4 to 7, essential to capture late reactions. Scoring follows ICDRG criteria: negative shows no reaction, irritant reactions show glazed erythema or pustules, 1+ shows faint erythema only, 2+ shows erythema with infiltration and papules, 3+ adds vesicles, and 4+ shows a bullous reaction.

### Interpretation Pitfalls

The clinician must distinguish relevant allergy from past sensitization without current clinical relevance. "Angry back" (excited skin syndrome) produces false positives from generalized hyperreactivity. False negatives occur from insufficient allergen concentration, inadequate occlusion, or concurrent immunosuppression. Irritant reactions can mimic weak positives. Patch test results must always be correlated with clinical history and exposure.

### Patient Counseling

After patch testing, patients should receive written allergen avoidance sheets (the ACDS CAMP database is a valuable resource), identification of allergen synonyms and hidden sources, discussion of cross-reactors, and safe product lists based on their specific test results.

## Special Forms of Contact Dermatitis

### Systemic Contact Dermatitis

When a sensitized patient has systemic exposure to an allergen (for example, ingested nickel or a systemic drug), the result can be generalized eczema, baboon syndrome (symmetric flexural erythema), or flare at previously affected sites.

### Photoallergic Contact Dermatitis

In this form, the hapten requires UV activation to become allergenic. Photo-patch testing is needed for diagnosis. Common photosensitizers include sunscreens (oxybenzone, octocrylene), NSAIDs, and fragrances.

### Contact Urticaria

This is an immediate (type I) hypersensitivity to a contactant, producing a wheal-and-flare response within minutes. Latex is the classic example, and food proteins can cause protein contact dermatitis. Contact urticaria can progress to anaphylaxis.

### Occupational Contact Dermatitis

This is the most common occupational skin disease, with high-risk occupations including healthcare work, hairdressing, construction, food service, and cleaning. Prevention centers on proper glove selection, barrier creams, and workplace modifications. Documentation and reporting carry medicolegal implications.

## Histopathology

ACD shows spongiotic dermatitis with epidermal intercellular edema. In acute cases, there is marked spongiosis, intraepidermal vesicles, and a dermal perivascular lymphocytic infiltrate with eosinophils. Chronic cases show acanthosis, minimal spongiosis, and fibrosis. ICD is more variable, sometimes showing epidermal necrosis, neutrophilic infiltrate, and less spongiosis than ACD. Histopathology alone cannot reliably distinguish ACD from ICD.

## Management

Identifying and avoiding the causative agent is the most important step. Acute disease is treated with cool compresses, topical corticosteroids (medium to high potency), and a short course of oral prednisone for severe or widespread ACD. Chronic disease requires emollients, topical corticosteroids, and topical calcineurin inhibitors for maintenance. Hand dermatitis benefits from barrier repair, cotton-lined gloves, and avoidance of wet work. For refractory hand eczema, alitretinoin (an oral retinoid approved in the EU) is an option.

<image>Patch testing procedure illustrated: (A) Finn chambers loaded with allergens applied to the upper back, (B) patch test reading at 48 hours showing negative, irritant, 1+, 2+, and 3+ reactions side by side with labeled scoring, (C) clinical example of allergic contact dermatitis to nickel showing eczematous dermatitis at the periumbilical area corresponding to a jean button.</image>

<image>Immunologic diagram of type IV delayed hypersensitivity in allergic contact dermatitis. Show the sensitization phase with hapten penetrating the stratum corneum, binding to a carrier protein, uptake by Langerhans cells, migration to lymph node, and T-cell priming. Then show the elicitation phase with re-exposure, local antigen presentation, recruitment of memory T cells, and the resulting spongiotic dermatitis. Include timeline markers (10-14 days for sensitization, 24-72 hours for elicitation).</image>

<image>Clinical distribution pattern map of common allergic contact dermatitis presentations: eyelid dermatitis (cosmetics, nail polish transfer), dorsal hand eczema (occupational), periumbilical (nickel), dorsal foot (shoe allergens), wrist (watch/jewelry), and ear (earrings). Use an anatomic diagram with labeled arrows pointing to each site with the corresponding common allergens listed.</image>

## Clinical Pearls

Always perform a delayed reading (day 4 to 7) of patch tests; up to 30 percent of positive reactions are missed if only read at day 2. Corticosteroid allergy should be suspected when eczema worsens with topical steroid use; tixocortol pivalate and budesonide are the screening allergens. Methylisothiazolinone (MI) is a preservative in cosmetics, wet wipes, and industrial products; sensitization rates skyrocketed in the 2010s, leading to regulatory restrictions in Europe. The dimethylglyoxime test is a bedside test for nickel content in metal objects — it turns pink if nickel is present. Eyelid dermatitis in women often results from allergens in nail polish (tosylamide/formaldehyde resin) transferred by touching the face — test the hands and nails, not just the eyelids. Systemic contact dermatitis to nickel can be provoked by high dietary nickel intake; a low-nickel diet may benefit some patients with chronic hand eczema and nickel allergy.

## References
- Alinaghi F, Bennike NH, Egeberg A, Thyssen JP, Johansen JD. Prevalence of contact allergy in the general population: a systematic review and meta-analysis. Contact Dermatitis. 2019;80(2):77-85.
- Fonacier L, Bernstein DI, Pacheco K, et al. Contact dermatitis: a practice parameter update 2015. J Allergy Clin Immunol Pract. 2015;3(3 Suppl):S1-S39.
- Nassau S, Fonacier L. Allergic contact dermatitis. Med Clin North Am. 2020;104(1):61-76.
- DeKoven JG, Warshaw EM, Belsito DV, et al. North American Contact Dermatitis Group patch test results: 2019-2020. Dermatitis. 2023;34(2):90-104.