Contact Dermatitis: Irritant vs Allergic, Patch Testing, and Hand Eczema
Atopic dermatitis is the eczema you inherit — written into a leaky skin barrier from birth. Contact dermatitis is the eczema you catch: the skin meets something in the outside world and reacts. Sometimes that something simply corrodes the skin by brute chemistry — no immune system required. Other times a single molecule teaches the immune system a grudge it will hold for life, so that every future contact triggers a delayed, itching flare. Telling these two apart, and above all naming the culprit, is what turns a chronic, career-threatening rash into a solvable problem.
A 34-year-old hairdresser comes in with her hands held out, palms up, almost apologetically. The skin across her fingers and wrists is red, cracked, and weeping, the fingertips split into painful fissures. It began months ago as dryness she blamed on "too much washing," but now the flares are relentless and she is missing shifts. Two problems are tangled together on those hands. The daily wet work — shampoos, water, bleach — has stripped her skin barrier raw by pure chemistry: irritant contact dermatitis. But layered on top is something the barrier breach let in: she has become allergic to a specific chemical in the dyes, and now even a trace sets off a delayed, spreading eruption. One rash, two mechanisms — and until someone patch-tests her, no one knows which chemical is ending her career.
Two eczemas that look alike and behave nothing alike
The same red, itchy, weeping patch can arrive by two utterly different routes. Both irritant and allergic contact dermatitis produce eczematous skin — redness, scaling, itch, and, when acute, tiny blisters and weeping. Under the microscope both show spongiosis (fluid pushing skin cells apart). But the machinery behind them is different in kind, not degree. Irritant contact dermatitis is non-immunological: a chemical or physical agent directly damages the skin, and the reaction is dose-dependent — enough exposure will irritate anyone's skin, no prior contact required. Allergic contact dermatitis is immunological: a delayed, T-cell-mediated hypersensitivity that only appears in people who have first been sensitised, and thereafter reacts to even a tiny amount. Getting the mechanism right is not academic — it decides whether the fix is barrier protection or detective work to name a single molecule.
Irritant contact dermatitis: brute chemistry, no immunity
Irritant contact dermatitis is by far the commoner of the two, and it is the classic occupational skin disease. The agent — a detergent, solvent, acid, alkali, or simply repeated water and friction — strips the lipids out of the stratum corneum, dissolves the barrier, and injures keratinocytes directly. The damaged skin cells release inflammatory signals, and the visible dermatitis follows. There is no memory here and no allergen: the response scales with how much and how often the skin is exposed. "Wet work" is the arch-offender — hairdressers, nurses, cleaners, caterers, metalworkers, anyone whose hands are wet, gloved, and scrubbed all day. Weak irritants cause a slow, cumulative dryness and cracking over weeks; a strong one (a splash of concentrated alkali) can burn on first contact. Because the barrier is already the problem, people with atopic dermatitis — whose barrier is genetically leaky, as covered in the Atopic Dermatitis chapter — are far more vulnerable to irritant hand eczema.
Allergic contact dermatitis: the immune system holds a grudge
This is a textbook type IV delayed hypersensitivity — the immunological heart of the topic. Allergic contact dermatitis unfolds in two acts. First, sensitisation: a small, fat-soluble chemical (a hapten — too tiny to be seen by the immune system on its own) penetrates the skin and binds to a self-protein, creating a new complex the body has never met. Langerhans cells and other dendritic cells engulf it, travel to the draining lymph node, and present it to naïve T cells, generating a population of memory T cells primed against that exact chemical. This first exposure causes no rash — it silently arms the immune system, and can take days to weeks. Then, elicitation: on any later contact, those memory T cells recognise the hapten in the skin, flood back, and release cytokines that drive the eczematous inflammation. Crucially this is a delayed reaction — the flare appears 24–72 hours after contact, not within minutes. That delay is the fingerprint of type IV hypersensitivity, and it is exactly why the culprit is so often missed: by the time the rash blooms, the exposure is two days in the past.
Think of the two eczemas as two ways a doorman reacts to a troublemaker. The irritant is like sandpaper dragged across the door — it damages the wood directly, and the more you rub the worse it gets; no one had to recognise anything. The allergen is like a face the doorman was once shown on a wanted poster: the very first time, nothing happens — he just memorises it. But every time that face returns, he raises the alarm and the whole security team storms in, even for a fleeting glimpse. Irritation is friction on the surface; allergy is a memorised grudge that answers the smallest reminder with a disproportionate, and deliberately delayed, response.
The single most useful contrast for exams and clinic: irritant reactions are dose-dependent and hit anyone with enough exposure, while allergic reactions are all-or-nothing and hit only the sensitised — but then react to a trace. So a distribution that spares no one on a ward points to an irritant; a sharply demarcated rash under a nickel watch strap, or exactly where a fragrance was dabbed, points to an allergen. And because sensitisation is lifelong, an allergen named once is an allergen the patient must avoid forever.
The usual suspects: who sensitises whom
A handful of allergens account for most allergic contact dermatitis, and they are worth knowing by name. Nickel is the classic — the metal in cheap jewellery, watch straps, belt buckles, and jean studs — and the commonest contact allergen worldwide, especially in women. Fragrances and preservatives (such as the methylisothiazolinones and formaldehyde-releasers hidden in cosmetics, wet wipes, and shampoos) are the great occult sensitisers. Rubber accelerators explain glove dermatitis — a cruel trap, because the very gloves worn to protect irritated hands can sensitise them. Then, the twist every clinician must respect: topical medicaments themselves. Neomycin (in over-the-counter antibiotic creams) is a frequent culprit, and so are the preservatives and even the active drug in many prescriptions. Most striking of all, topical corticosteroids — the drugs we reach for to treat dermatitis — can themselves be contact allergens. Suspect it when an eczema mysteriously worsens, or simply refuses to clear, under steroid treatment.
The location often names the allergen. A rash confined to the earlobes and under a necklace clasp: nickel. Eyelid dermatitis with no obvious cause: frequently a nail-varnish or fragrance allergy transferred by the fingers. A rash under the waistband exactly tracing an elasticated seam: rubber. Weeping around a chronic leg ulcer that has been dressed for weeks with an antibiotic ointment: neomycin. And the diagnostic paradox — an inflamed patch that flares wherever a steroid cream was applied, worse rather than better: a corticosteroid allergy. Each of these is confirmed the same way — not by guessing, but by patch testing.
- Irritant contact dermatitis is non-immunological, dose-dependent direct damage — anyone can get it with enough exposure.
- Allergic contact dermatitis is a type IV, T-cell-mediated delayed hypersensitivity needing prior sensitisation.
- Sensitisation is silent and lifelong; elicitation then flares 24–72 hours after even a trace of contact.
- Top allergens: nickel, fragrances, preservatives, rubber accelerators, and topical medicaments (neomycin — and steroids themselves).
- Hand eczema in wet-work occupations is frequently a mix of irritant and allergic mechanisms on the same skin.
- A steroid-treated eczema that worsens or won't clear should raise suspicion of corticosteroid contact allergy.
Diagnosis: patch testing, and why it isn't prick testing
The test must match the immunology — and type IV is slow, so the test is slow. To find the allergen behind allergic contact dermatitis, you use patch testing — and its whole design mirrors the delayed, T-cell nature of the reaction. A standardised battery of suspected allergens is applied in small chambers taped to the back, left in place for 48 hours, then removed. The skin is read at 48 hours and again at 72–96 hours, looking for a localised eczematous reaction — redness, papules, sometimes blistering — exactly under one chamber. That reading window matters: type IV responses take days to develop, so an early-only reading misses them. This is fundamentally different from the skin prick test used for type I (immediate) allergy, which is described in the Principles / Immunology chapter. Prick testing introduces allergen into the superficial skin and reads a wheal-and-flare in 15–20 minutes, detecting IgE-mediated, mast-cell reactions like hay fever or food allergy. Patch testing detects T-cell memory; prick testing detects IgE. Using the wrong one answers the wrong question.
Management pillar one: identify and avoid — the definitive treatment
Every drug in this article is secondary to one principle: the definitive treatment of contact dermatitis is removing the cause. No cream matches avoidance. For allergic contact dermatitis, that means naming the allergen by patch testing and then rigorously eliminating it — switching to nickel-free fasteners, fragrance-free products, or a different glove chemistry. For irritant dermatitis, it means reducing the exposure and, critically, restoring the barrier: this is where the pharmacology begins to matter. Barrier protection with gloves (the right material, not a sensitising one) and generous, frequent emollients rebuild the stratum corneum's lipid film so it can once again keep irritants out. Emollients here play the same role they do in the Atopic Dermatitis chapter — they are not a cosmetic afterthought but the structural foundation of every eczema plan, the substrate on which the anti-inflammatory drugs act.
Management pillar two: the drugs that calm the flare
Avoidance treats the cause; drugs treat the inflammation that is already burning. For an active flare, topical corticosteroids are the first-line anti-inflammatory — the same drug class detailed in the Topical Corticosteroids chapter. Potency is matched to the site: a potent or very potent steroid for the thick skin of the palms, a milder one for thin skin, and never a strong steroid long-term on the face. For the face, eyelids, and flexures — where steroids risk atrophy — the topical calcineurin inhibitors, tacrolimus (Tacrolimus) ointment and pimecrolimus (Pimecrolimus) cream, are the steroid-sparing alternative covered in the Calcineurin Inhibitors chapter; they suppress T-cell cytokine production without thinning the skin, which suits the T-cell-driven mechanism of allergic contact dermatitis especially well. When the eczema is widespread and severe, a short, tapering course of oral corticosteroids can break an acute flare — but only as a bridge, never as chronic control. And here the article's own paradox returns: because topical steroids can themselves sensitise, an eczema that flares under steroid treatment should prompt patch testing to the corticosteroid itself rather than simply escalating the potency.
Chronic hand eczema: when topical measures aren't enough
Chronic hand eczema is the great real-world burden of this topic — a leading cause of occupational disability, keeping hairdressers, healthcare workers, and cleaners off work and, too often, out of their trade. When avoidance, emollients, and topical steroids fail to control it, two systemic options step up. Alitretinoin (Alitretinoin) — an oral retinoid (9-cis-retinoic acid) — is specifically licensed for severe chronic hand eczema unresponsive to potent topical steroids; like all oral retinoids it is a powerful teratogen, demanding strict pregnancy prevention and monitoring, a principle shared with isotretinoin in the Acne chapter. Phototherapy — narrowband UVB or hand-and-foot PUVA — is the other established route, using ultraviolet light to dampen the cutaneous T-cell response. Beyond these, severe refractory cases may reach for the broader systemic immunosuppressants (ciclosporin, azathioprine, methotrexate) or, increasingly, newer targeted agents. But every one of these is a fallback: the whole thrust of good contact-dermatitis care is to make systemic therapy unnecessary by naming and removing the trigger first.
- Patch testing (48h application, read at 48h and 72–96h) finds type IV allergens; prick testing finds type I IgE allergy — don't confuse them.
- Definitive treatment is identifying and avoiding the irritant/allergen; no drug substitutes for removing the cause.
- Barrier protection (gloves) and frequent emollients rebuild the skin barrier — the foundation of every plan.
- Topical corticosteroids treat flares; topical calcineurin inhibitors (tacrolimus, pimecrolimus) spare the face and flexures.
- Severe chronic hand eczema: alitretinoin (oral retinoid, teratogenic) or phototherapy after topical measures fail.
- Occupational hand eczema is a major cause of work disability — prevention and barrier care are as important as any drug.
- Treating an unresponsive eczema by climbing to stronger and stronger steroids instead of patch-testing — including for a steroid allergy itself.
- Ordering a skin prick test to hunt for a contact allergen. Prick tests detect immediate type I (IgE) allergy, not the delayed type IV reaction of contact dermatitis.
- Prescribing creams while ignoring the exposure — leaving the patient in the same wet-work or jewellery that keeps re-triggering the rash.
A metalworker develops an itchy, well-demarcated eczema on the backs of both hands. It flares 2–3 days after his shifts and settles on holiday. Topical steroids help only partly. What is the single most useful next investigation to identify the cause?
- Contact dermatitis is caught, not inherited: irritant (non-immunological, dose-dependent direct damage) versus allergic (type IV, T-cell delayed hypersensitivity needing prior sensitisation).
- Allergic disease flares 24–72 hours after contact; common allergens are nickel, fragrances, preservatives, rubber, and topical medicaments including neomycin and even corticosteroids.
- Diagnose the allergen by patch testing (delayed reading) — not prick testing, which is for immediate type I allergy.
- Management: identify and avoid the cause (definitive), barrier protection + emollients, topical steroids for flares and calcineurin inhibitors for the face; severe chronic hand eczema may need alitretinoin or phototherapy.
- Rook's Textbook of Dermatology — Contact dermatitis: irritant and allergic.
- Katzung Basic & Clinical Pharmacology — Dermatologic pharmacology: topical corticosteroids and immunomodulators.
- Wolverton SE. Comprehensive Dermatologic Drug Therapy — Retinoids (alitretinoin) and topical calcineurin inhibitors.
- British National Formulary (BNF) — Topical corticosteroids, tacrolimus/pimecrolimus, and alitretinoin.
- Johansen JD, et al. European Society of Contact Dermatitis guideline for diagnostic patch testing.
- Ruzicka T, et al. Oral alitretinoin for severe chronic hand eczema refractory to topical corticosteroids. British Journal of Dermatology.
