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Psoriasis has no single cause. How genes, the interleukin-23 and interleukin-17 immune circuit, skin memory, body weight, and everyday triggers combine, and what that means for treatment.
What are the roots of psoriasis? The question often carries the hope that one hidden cause, once found and removed, will make the plaques disappear for good. Psoriasis does not work that way. It grows from inherited genes, an immune circuit that has learned to keep itself running, skin cells that actively join the inflammation, and metabolic and environmental influences that can push a susceptible person over the threshold. Understanding how these roots connect explains why psoriasis runs in families, why it returns in the same places, and why the most effective treatments target specific immune signals rather than a single culprit.
An estimated 125 million people worldwide have psoriasis [1]. In North America, prevalence is estimated at about 1.5 percent, and the condition is more common in high-income countries and in regions with older populations [2]. Psoriasis is a chronic immune-mediated disease rather than an infection. It cannot be caught from another person, and it is not caused by poor hygiene.
The most common form, chronic plaque psoriasis, is described in current reviews as the consequence of genetic susceptibility combined with environmental triggers such as streptococcal infection, stress, smoking, obesity, and alcohol consumption [3]. Immunological and genetic research has identified interleukin-17 and interleukin-23 as key drivers of the disease [3]. Each root contributes a different amount in different people, which is one reason two patients with the same diagnosis can follow very different courses.
Psoriasis is strongly heritable. In a Danish questionnaire study of 10,725 twin pairs, the chance of psoriasis in the twin of an affected person was 33 percent in identical twins and 17 percent in non-identical twins, and genetic factors explained approximately 68 percent of the variation in susceptibility [4]. The same numbers carry a second message. Identical twins share essentially all of their genes, yet most identical twins of an affected person in that study did not report the disease. Inheritance alone cannot be the whole explanation.
Nor is inheritance carried by one gene. Sixty-five regions of the genome have been linked to psoriasis risk in people of European ancestry, and the largest contribution comes from HLA-C*06:02, a variant of an immune gene that is also known by its older name, HLA-Cw6 [5]. HLA molecules display protein fragments to T cells, the white blood cells that coordinate targeted immune responses. Variants in a related gene, ERAP1, which trims those fragments, confer risk only in people who also carry the HLA-C risk variant [5]. Why variants like these may have persisted in human populations is explored in the article on psoriasis during plague time.
Age of onset offers a further clue. A 1985 study of 2,147 patients described two peaks of onset, one in the teens or early twenties and another around age 60 [6]. In the subset that was tissue typed, the HLA-Cw6 marker was found in 85.3 percent of early-onset patients and 14.7 percent of late-onset patients. A parent was affected in about half of early-onset patients and in none of the late-onset group [6]. The study is old and its figures should not be read too precisely, but the pattern helps explain why some people have a strong family history and others have none.
Rare forms can behave differently. Generalized pustular psoriasis, a rare and severe subtype, was linked in Tunisian families to recessive mutations in IL36RN, a gene whose product normally restrains inflammatory signalling [7]. These mutations are found in only about 25 percent of pustular psoriasis cases [5], and research now separates pustular forms from chronic plaque disease [3]. For the common plaque form, no single mutation explains the disease.
The engine of plaque psoriasis is a signalling circuit built around two proteins, interleukin-23 and interleukin-17. An early clue came from a 2004 study in which messenger RNA for the p19 subunit, the part unique to interleukin-23, was 22.3-fold higher in plaques than in nearby uninvolved skin, while a subunit specific to the related protein interleukin-12 was not increased [8]. In current models, interleukin-23 sustains T cells that release interleukin-17, which in turn drives inflammation and the rapid skin cell growth that builds a plaque [1].
Some of the strongest evidence for this circuit comes from treatment. Drugs that block interleukin-17 or the p19 subunit of interleukin-23 are effective in moderate to severe plaque psoriasis [1], and immune targeting of these proteins and of tumour necrosis factor has transformed the care of severe disease [3]. When interrupting a pathway clears the skin, that pathway is very likely part of the cause. How these agents are selected is described on the page covering biologic and advanced small molecule therapy.
What first switches the circuit on is not settled. Psoriasis has long been suspected of being autoimmune, meaning that T cells react to the body's own proteins, and two candidate targets have been identified. In one study, two-thirds of patients with moderate to severe plaque psoriasis carried T cells specific for LL37, an antimicrobial peptide overexpressed in psoriatic skin, and the presence of these cells in the blood correlated with disease activity [9]. In another, a T cell receptor taken from an HLA-C*06:02-positive patient recognized melanocytes, the pigment cells of the skin, through a protein called ADAMTSL5 [10].
These findings offer a plausible link between the main risk gene and inflammation confined to the skin. They remain research observations, however, and neither target is currently used to diagnose or treat psoriasis.
The skin is a participant in psoriasis rather than a passive victim. Genomic regions linked to the disease implicate skin barrier processes as well as immune ones, and CARD14, a gene that encodes an activator of inflammatory NF-kB signalling, carries variants associated with both rare and common forms of psoriasis [5].
The skin also keeps a record. Effective treatment heals plaques without scarring, yet psoriasis typically returns in the same locations. A study of resolved lesions after narrowband UVB or long-term biologic treatment found that tissue-resident memory T cells had been retained in the epidermis, and that they could still produce interleukin-17A or interleukin-22 when stimulated in the laboratory, even after as long as six years of tumour necrosis factor inhibition [11]. Clear skin is therefore not the same as absent disease, a distinction examined in the article on whether psoriasis can be cured.
Body weight is among the best-studied contributors. Mendelian randomization, a method that uses inherited variants as a natural experiment and is less vulnerable to confounding than ordinary observational research, indicated that each one-unit increase in body mass index raises the odds of psoriasis by 9 percent [12]. Genetic risk for psoriasis showed little effect on body mass index in the reverse direction, which suggests that excess weight contributes to psoriasis rather than only following from it. The cohorts had limited ethnic diversity, and misreporting or misdiagnosis of psoriasis was possible [12].
Weight loss can help without removing the disease. In a randomized trial of 303 overweight or obese patients with moderate to severe psoriasis who were receiving systemic treatment, a 20-week diet and exercise plan produced a median 48 percent reduction in the Psoriasis Area and Severity Index (PASI), compared with 25.5 percent after information alone [13]. A smaller trial of 60 patients found that about 15 kg of additional weight loss on a low-energy diet improved quality of life, while the 2-point PASI difference narrowly missed statistical significance [14]. Weight management is a useful complement to treatment, not a replacement for it.
Smoking follows a similar direction with weaker proof. A meta-analysis found that current smokers had higher odds of psoriasis (pooled odds ratio 1.78), and three incidence studies suggested a possible dose effect [15]. The data are observational and possible publication bias was noted, although stopping smoking is advisable for many other reasons.
Diet is the question raised most often in clinic. Celiac disease is more prevalent in people with psoriasis, and the two conditions appear to share genetic and inflammatory pathways [16]. Any benefit from a gluten-free diet, however, appears limited to patients with celiac markers, as reviewed in the article on gluten and skin disease. Routine elimination diets are not a treatment for psoriasis.
A useful distinction separates roots from triggers. Roots are the underlying genetic and immune features that make the skin prone to psoriasis. Triggers are the events that provoke a first episode or a flare in someone already susceptible. Removing a trigger may reduce flares, but it does not remove the susceptibility, which is why psoriasis can return long after a trigger has passed.
The best-documented example is guttate psoriasis, an abrupt eruption of small, drop-shaped plaques. In a study of 111 patients with a sudden onset or worsening of psoriasis, serological evidence of recent streptococcal infection was present in 58 percent of those tested with acute guttate disease. Streptococcus pyogenes was isolated from 26 percent of them, compared with 7 percent of controls [17].
Recognized triggers include the following:
Medication deserves a specific caution. The link can be hard to recognize because the delay between starting a drug and the onset of psoriasis can be long, and lesions may persist after the drug is stopped [20]. A suspected medication should be reviewed with the prescribing physician rather than discontinued independently. More detail is available in the article on drug-induced psoriasis.
Not a personal failing. Psoriasis arises largely from inherited immune biology. A flare does not mean that a person has done something wrong, and no single food, toxin, or habit explains the disease.
Treatment targets the roots that can be reached. Genes cannot be changed, but the immune circuit can be interrupted, and weight, smoking, and some triggers can be addressed. Current expert guidance favours treating early, identifying associated conditions, supporting lifestyle changes, and personalizing treatment [3].
A genetic test will not answer the question. Direct-to-consumer genetic risk profiles are likely to be insufficiently precise or consistent to offer substantial clinical utility, and they are vulnerable to misunderstanding [5]. A family history and a careful examination remain more useful in everyday care.
The same roots can reach beyond the skin. Psoriasis is associated with depression, cardiometabolic disease, and psoriatic arthritis [3]. Joint pain, prolonged morning stiffness, or a swollen finger or toe should be reported rather than attributed to age or activity.
A consultation at the Centre for Medical and Surgical Dermatology allows family history, personal triggers, and related health conditions to be reviewed together, and a treatment plan to be matched to the severity and pattern of the disease.
This article is intended for educational purposes and does not replace professional medical advice. Please consult your dermatologist for personalized recommendations.
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