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A complete guide to psoriasis — the IL-17/IL-23/TNF-α immune axis that drives keratinocyte hyperproliferation, the five clinical types (plaque, guttate, inverse, pustular, erythrodermic), distinguishing psoriasis from seborrheic dermatitis and eczema, the treatment hierarchy from emollients and topical corticosteroids through phototherapy (narrowband UVB) to conventional systemic drugs (methotrexate, cyclosporine, acitretin) and biologics (TNF-α, IL-17, IL-23 inhibitors), psoriatic arthritis, and triggers that drive flares.
· By MedSpot Editorial · 5 min read
Psoriasis affects approximately 2–3% of the global population — roughly 125 million people. It is a chronic, immune-mediated inflammatory disease that primarily affects the skin but has systemic manifestations including psoriatic arthritis, increased cardiovascular risk, and metabolic syndrome association. Here is the complete evidence-based guide.
Psoriasis is driven by a dysregulated adaptive immune response — not a primary keratinocyte defect. The cascade:
TNF-α plays a central amplifying role — produced by macrophages and T cells in psoriatic lesions, it amplifies the IL-17 response and directly drives keratinocyte activation. This is why anti-TNF biologics were the first to demonstrate efficacy in psoriasis.
IL-23 is the master upstream regulator — blocking IL-23 (the p19 subunit) produces sustained remission in many patients because it cuts off the driver of Th17 polarization. The IL-23 inhibitors (risankizumab, guselkumab, tildrakizumab) have emerged as the highest-efficacy biologics in psoriasis.
Psoriasis has strong genetic heritability — roughly 65–70% concordance in identical twins. The strongest genetic association is HLA-Cw6 (the PSORS1 locus on chromosome 6p), which increases risk 10-fold. HLA-Cw6 positive patients more commonly have guttate-onset disease and a strong response to IL-17 inhibitors.
The most common type — ~90% of psoriasis cases. Characterized by:
Small, drop-shaped plaques (guttate = "drop") scattered over the trunk and extremities. Often triggered by streptococcal pharyngitis (throat swab + ASO titer useful). Most common first presentation in children and young adults. May resolve spontaneously; may evolve to plaque psoriasis.
Affects skin folds — axillae, inguinal folds, inframammary area, gluteal cleft. Minimal scale (maceration in folds removes it); smooth, glazed erythema. Resembles intertrigo, seborrheic dermatitis, or candidiasis — diagnosis often requires biopsy.
Sterile pustules on an erythematous base. Can be localized (palmoplantar pustulosis — palms and soles) or generalized (von Zumbusch — rare, acute, potentially life-threatening with fever and systemic involvement). Generalized pustular psoriasis requires emergency management.
Widespread erythema covering >90% of body surface area — may arise from unstable plaque psoriasis. Potentially life-threatening from thermoregulatory failure and protein loss. Emergency dermatology management required.
| Feature | Psoriasis | Seborrheic Dermatitis | Atopic Dermatitis |
|---|---|---|---|
| Scale | Thick, silvery-white, adherent | Thin, yellowish, greasy | Fine, dry |
| Borders | Sharp, well-demarcated | Diffuse | Diffuse |
| Distribution | Extensor, scalp, nails | Sebaceous areas (T-zone, scalp) | Flexural |
| Itch | Moderate | Moderate | Severe |
| Nail changes | Yes (pitting, onycholysis) | No | Rarely |
| Koebner | Yes | No | No |
Emollients: All psoriasis patients need regular emollient application — softens and partially removes scale, reducing the barrier function deficit. Thick creams and ointments preferred.
Topical corticosteroids: First-line anti-inflammatory. Moderate to potent TCS (triamcinolone 0.1%, betamethasone valerate 0.1%, clobetasol 0.05% for thick plaques). Applied daily to active plaques; taper to 2–3×/week after clearance; not for indefinite daily use (skin atrophy, tachyphylaxis).
Vitamin D analogues (calcipotriol/calcitriol): Inhibit keratinocyte proliferation via VDR (vitamin D receptor); reduce Th17 cytokine signaling. Used alone or in combination with TCS. Dovobet (calcipotriol + betamethasone) — the fixed-dose combination gel — is more effective than either component alone; approved for scalp and body use.
Calcineurin inhibitors: Tacrolimus and pimecrolimus for inverse psoriasis in skin folds — appropriate where TCS atrophy is a concern; less effective than TCS for plaque disease.
Coal tar: Antiproliferative and anti-inflammatory; older agent but effective for scalp and body. Cosmetically less acceptable.
Phototherapy — narrowband UVB (NBUVB):
Conventional systemics:
Anti-TNF agents: Adalimumab, etanercept, infliximab. First-generation biologics; well-established long-term safety data. PASI 75 rates ~60–70%.
Anti-IL-17 agents: Secukinumab (Cosentyx), ixekizumab (Taltz), bimekizumab (Bimzelx — also blocks IL-17F). Faster onset than anti-TNF; PASI 90 rates ~60–70%; among the highest-efficacy biologics available.
Anti-IL-23 agents (p19): Risankizumab (Skyrizi), guselkumab (Tremfya), tildrakizumab (Ilumya). Every 8–12 week maintenance dosing after induction; PASI 90 rates 70–80%; PASI 100 (complete clearance) in 40–50%. Among the highest single-agent efficacy in psoriasis.
Up to 30% of psoriasis patients develop psoriatic arthritis — inflammatory arthritis that can cause permanent joint damage. Screening questions:
Early diagnosis and treatment with DMARDs or biologics prevents joint damage. Refer to rheumatology if suspected.
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