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Psoriasis is a hereditary disorder of skin characterized by chronic scaling papules and plaques in a characteristic distribution, largely at sites of repeated minor trauma. The HLA types most frequently associated with psoriasis are HLA-Cw6, -B13, -B17, -Bw16, -B37, and -DR7.
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INSIGHT 
It can be difficult to distinguish between atopic dermatitis and psoriasis in infancy. If family history and cutaneous findings are not helpful, one hint is atopic dermatitis usually spares the diaper area and psoriasis favors that location.
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AGE 10% have onset of lesions before age 10, and up to 33% have onset by age 20 years.
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GENDER Slight predominance F > M.
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PREVALENCE 2% of the world's population. United States and Canada: 4% to 5% of the population.
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RACE Low incidence in Asians, Africans, African Americans, American Indians, and Japanese relative to Caucasians.
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OTHER FEATURES Multifactorial inheritance. Minor trauma is a predisposing factor (45% of patients) in eliciting lesions (Koebner's phenomenon). Infection (particularly streptococcal) also plays a role. Many episodes of psoriasis—and particularly guttate psoriasis— follow sore throats or upper respiratory infections. Stress, cold weather, hypocalcemia, and lack of sunlight exposure aggravate the condition. Certain drugs (lithium, interferon, β-blockers, alcohol, antimalarials, corticosteroid withdrawal, and paradoxically anti-TNFα biologics) can also precipitate psoriasis.
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Psoriasis is likely a polygenic disease caused by the inappropriate activation of T cells (the adaptive immune system) as well as abnormal keratinocyte proliferation (the innate immune system). A predominantly TH1 inflammatory milieu underlies the chronic inflammation of psoriasis. Several cytokines including interferon-α, TNF-α, interleukin 23, and interleukin 17 are known to play critical roles in the initiation and prolongation of the inflammation in psoriasis that drives T-cell recruitment and increased keratinocyte proliferation.
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ONSET OF LESIONS Usually slowly over the course of months but may be sudden as in acute guttate psoriasis and generalized pustular psoriasis (von Zumbusch).
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SKIN SYMPTOMS Pruritus is reasonably common, especially in scalp and anogenital psoriasis.
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CONSTITUTIONAL SYMPTOMS In 5% of cases, psoriasis can be associated with arthritis, fever, and/or an “acute illness” syndrome (weakness, chills, fever) with generalized erythroderma.
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TYPE Well-delineated, erythematous, thickened plaques with a characteristic silvery-white scale (Fig. 4-1A). Removal of scale results in the appearance of miniscule blood droplets (Auspitz sign).
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COLOR Salmon pink to red.
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SIZE Can range from pinpoint 1-mm papules to large ...