Eczematous dermatitis
- Lesion
Eczema is itchy, red, oozing skin that becomes thick and scaly when it lasts. It has many causes, from allergies to weak skin barriers.
What it is
Eczematous dermatitis is a reaction pattern rather than a single disease. Acute lesions are red, oozing vesicles with spongiosis under the microscope; persistent scratching turns them into thick, scaly, lichenified plaques. The main types are allergic contact dermatitis (type IV hypersensitivity), irritant contact dermatitis (direct damage), atopic dermatitis (barrier defect plus Th2 inflammation), drug-related and photo-eczematous eruptions.
Step by step
- A hapten penetrates the skin and binds a host protein.
- Langerhans cells take it up and carry it to regional lymph nodes.
- Naive T cells become sensitised memory T cells (no reaction on first exposure).
- On re-exposure, memory T cells home to the skin and release cytokines within one to two days.
- Keratinocytes swell apart (spongiosis), vesicles form, and itch-scratch cycles thicken the skin.
Eczema types
| Type | Trigger | Mechanism | Clue |
|---|---|---|---|
| Allergic contact | Nickel, urushiol, fragrances, rubber, topical drugs | Type IV (T cell) | Shape matches the contact; positive patch test |
| Irritant contact | Detergents, solvents, wet work | Direct toxicity | Hands; no sensitisation needed |
| Atopic | Barrier defect, allergens, microbes | Th2, IgE, filaggrin loss | Flexures, childhood, atopy |
| Drug-related | Systemic drugs | Hypersensitivity | Widespread; improves on stopping the drug |
| Photo-eczematous | UV light plus a chemical | Photoallergic or phototoxic | Sun-exposed sites only |
Clinical notes
- Atopic dermatitis is often infected by S. aureus and can be complicated by eczema herpeticum (HSV).
- Occupational contact dermatitis is common in health workers (gloves, disinfectants).
Test yourself
Think of the answer, then tap the question to see it.
What is the key microscopic feature of acute eczematous dermatitis?
Spongiosis: intercellular oedema between keratinocytes, forming intraepidermal vesicles.
Which hypersensitivity type underlies allergic contact dermatitis?
Type IV, delayed, T cell-mediated hypersensitivity to a hapten.
Which gene defect is a major risk factor for atopic dermatitis?
Loss-of-function mutations in filaggrin (FLG).
Elsewhere in DurgiPedia
- Type IV hypersensitivity (delayed, T cells)
- Type I hypersensitivity (immediate, IgE)
- Th1, Th2, Th17, Tfh and Treg
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Eczema (MedlinePlus, US National Library of Medicine, 2025); Atopic dermatitis (MedlinePlus, US National Library of Medicine, 2025).
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Representative model, for educational purposes only.