Inflammatory and Reactive Dermatoses: Pattern Recognition and Pitfalls
Pattern-first approach
Inflammatory dermatopathology rewards a fixed algorithm: identify the reaction pattern at low power, then the dominant cell type, then distribution (superficial vs deep, perivascular vs periadnexal vs interstitial), then specific clues. The six patterns in the blueprint are interface, spongiotic, psoriasiform, vesiculobullous, granulomatous, and vasculopathic.
Concepts to master
Interface dermatitis divides into vacuolar (lupus, dermatomyositis, lichen sclerosus) and lichenoid (lichen planus, lichenoid drug, fixed drug). Lichen planus shows wedge-shaped hypergranulosis, saw-tooth rete ridges, and a band-like infiltrate; lupus shows periadnexal and perivascular extension, mucin, and thickened basement membrane. Spongiotic dermatitis: acute shows intraepidermal vesicles and eosinophils; chronic shows acanthosis and parakeratosis. Allergic contact and nummular eczema both show eosinophils, so distribution and history decide. Psoriasiform: psoriasis has regular acanthosis, thin suprapapillary plates, Munro microabscesses, and dilated papillary capillaries; pityriasis rubra pilaris shows checkerboard alternation; lichen simplex chronicus shows irregular acanthosis with fibrosis. Vesiculobullous disease: DIF is decisive, with intercellular IgG (pemphigus), linear IgG/C3 at the basement membrane (bullous pemphigoid), linear IgA (linear IgA disease), and granular IgA in dermal papillae (dermatitis herpetiformis). Granulomatous: naked (sarcoid), suppurative (infection, ruptured cyst), palisaded (granuloma annulare, necrobiosis lipoidica). Vasculopathic: leukocytoclastic vasculitis shows fibrinoid necrosis, nuclear dust, and RBC extravasation; occlusive mimics such as calciphylaxis lack true vasculitis.
Traps
Do not diagnose vasculitis without fibrinoid necrosis and nuclear dust. Do not miss infection behind any granulomatous or suppurative pattern. A spongiotic dermatitis with eosinophils is not automatically allergic contact.
Rule of thumb
Pattern + cell type + site = differential; DIF and stains confirm. A superficial perivascular infiltrate with focal spongiosis, eosinophils, and parakeratosis favors allergic contact dermatitis; the same infiltrate band-like with basilar vacuoles and mucin favors lupus.