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Eczema has no single cause. A dermatologist explains how genes, the skin barrier, type 2 immunity, skin microbes, and the environment combine, and why roots differ from triggers.
What are the roots of eczema? The question is usually asked in the hope that a single hidden cause, once found and removed, will make the disease disappear. Atopic dermatitis does not work that way. It grows from several roots at once: inherited genes, a weakened skin barrier, an immune system tilted toward allergic inflammation, an altered community of skin microbes, and the environment in which a person lives. Understanding how these roots intertwine explains why eczema behaves as it does, why it so often runs in families, and why the most effective treatments target biology rather than a single culprit.
Atopic dermatitis, the most common form of eczema, affects up to 20 percent of children and up to 10 percent of adults in high-income countries [1]. It is a chronic, relapsing inflammatory skin disease marked by dry skin, intense itch, and recurrent patches of inflammation. Most cases begin in infancy or early childhood, although onset in adulthood is increasingly recognized [1].
No single gene, food, or exposure causes atopic dermatitis in everyone. Current models describe a complex disease in which genetic susceptibility, skin barrier dysfunction, immune dysregulation, and environmental factors interact and reinforce one another [2]. Each root contributes a different amount in different people, which is one reason two patients with the same diagnosis can follow very different courses. A more technical account of these mechanisms is available in the overview of eczema pathology.
Eczema clusters in families, and a child with a parent who has eczema, asthma, or hay fever carries a higher risk of developing it. Twin studies indicate that a large share of susceptibility is inherited [2]. The inheritance is not carried by one gene, however. Dozens of genetic variants have been linked to atopic dermatitis, and many of them are involved in skin barrier formation or immune regulation [2].
The most important single genetic discovery came in 2006, when loss-of-function variants in the filaggrin gene (FLG) were identified as a major predisposing factor for atopic dermatitis [3]. Filaggrin is a protein that helps skin cells flatten and bind together in the outermost layer of the skin, and its breakdown products contribute to the natural moisturizing factor that keeps the surface hydrated. Up to 10 percent of people of European ancestry carry an FLG null variant [4].
Genes raise the odds; they do not decide the outcome. Many people who carry FLG variants never develop eczema, and most people with eczema do not carry them [4]. Genetic susceptibility raises the probability of disease without determining it. That distinction matters because it leaves room for the other roots, several of which can be influenced.
The outermost layer of the skin, the stratum corneum, functions like a brick wall. Flattened skin cells form the bricks, and a mixture of lipids, including ceramides, cholesterol, and free fatty acids, forms the mortar. In atopic dermatitis this wall is defective even in skin that looks normal: water escapes more easily, and irritants, allergens, and microbes penetrate more readily [2].
A leaky barrier does more than cause dryness. Allergens that cross damaged skin meet the immune system in an inflammatory setting, which favours sensitization. The dual-allergen exposure hypothesis proposes that exposure to food proteins through inflamed skin tends to promote allergy, while early exposure through the gut tends to promote tolerance [5]. This model helps explain why eczema usually precedes food allergy rather than the reverse, a relationship discussed in the article on food allergy and skin disease.
Beneath the barrier sits an immune system tilted toward type 2 inflammation, the same branch of immunity involved in asthma and hay fever. Two signalling proteins, interleukin-4 and interleukin-13, are central. They promote inflammation, suppress the production of barrier proteins such as filaggrin, and sustain the disease in a self-reinforcing loop [2]. Some of the strongest evidence for their role came from treatment: blocking their shared receptor with dupilumab produced rapid and marked improvement in adults with moderate to severe atopic dermatitis in early clinical trials [6].
Itch is not merely a side effect of this inflammation. Laboratory research has shown that interleukin-4 and interleukin-13 act directly on sensory nerves, sensitizing them to itch signals through a pathway that depends on the enzyme JAK1 [7]. Scratching then damages the barrier further, releases more inflammatory signals, and intensifies the itch. This itch-scratch cycle is one of the main reasons eczema is so persistent once it has become established.
Healthy skin hosts a diverse community of bacteria. In atopic dermatitis that diversity narrows, and the bacterium Staphylococcus aureus becomes more abundant. A longitudinal study in children found that the proportion of S. aureus rose sharply during flares and fell again with treatment, while overall bacterial diversity followed the opposite pattern [8]. Later work showed that more severe flares were dominated by S. aureus, whereas milder flares were associated with a greater share of Staphylococcus epidermidis [9].
Whether S. aureus is a cause or a consequence remains under study, and the most likely answer is both. A damaged, inflamed barrier favours its growth, and its toxins and enzymes in turn aggravate inflammation and barrier damage. The broader role of microbial communities in chronic skin disease is reviewed in the article on the skin microbiome and chronic skin conditions.
Human genes do not change within a few generations, yet the prevalence of atopic dermatitis increased over the second half of the twentieth century, most noticeably in industrialized countries [10]. That pattern points to the environment. In 1989, the observation that hay fever and infant eczema were less common in children with older siblings gave rise to the hygiene hypothesis, the idea that reduced early-life exposure to microbes may predispose to allergic disease [11]. The hypothesis has since been refined to emphasize microbial diversity rather than cleanliness itself, but its central observation, that the early-life environment shapes immune development, has held up.
Many other environmental factors have been associated with eczema, including urban living, cold and dry climates, low humidity, air pollution, and exposure to tobacco smoke [10]. Most of this evidence comes from observational studies, so association should not be mistaken for proof of causation. Several of these factors probably act by stressing an already vulnerable barrier rather than by creating the disease from nothing.
Domestic water hardness offers a well-studied example. In a population-based study of three-month-old infants in the United Kingdom, exposure to high domestic water calcium carbonate levels was associated with a significantly increased risk of atopic dermatitis, with a suggestion of a stronger effect in infants carrying FLG variants [12]. The finding illustrates how an environmental exposure and a genetic root can interact, although it does not establish that softening household water prevents eczema.
Diet is the environmental factor raised most often in clinic. Outside a confirmed food allergy, dietary restriction has not been shown to treat established eczema; the evidence is reviewed in detail in the article on whether diet can cure eczema.
If a defective barrier is an early root, protecting the barrier from birth might be expected to prevent eczema. Early results were encouraging. A randomized trial in Japan reported that daily moisturizer applied to high-risk newborns resulted in approximately 32 percent fewer cases of atopic dermatitis by 32 weeks of age [13].
Larger trials did not confirm the effect. The BEEP trial in the United Kingdom found that daily emollient use during the first year of life did not reduce eczema by age two in high-risk infants, and it was associated with more frequent skin infections [14]. The PreventADALL trial in Norway and Sweden found that neither regular skin emollients nor early complementary feeding reduced atopic dermatitis by 12 months of age in a general population of infants [15]. On current evidence, routine emollient use from birth cannot be relied upon to prevent eczema, although emollients remain essential for treating eczema once it has developed.
These results do not disprove the barrier theory. They show that the roots of eczema are deeper and more interconnected than a single intervention can reach, and that prevention strategies will likely need to account for genetic risk, immune development, and environment together. Research into early prevention continues.
A useful distinction separates the roots of eczema from its triggers. Roots are the underlying genetic, barrier, and immune features that make the skin prone to inflammation. Triggers are the day-to-day exposures that provoke a flare in skin that is already susceptible. Removing a trigger can reduce flares, but it does not remove the disease, which is why eczema returns even in people who carefully avoid every known irritant.
Common triggers include the following [2]:
Allergic contact dermatitis deserves separate mention. People with atopic dermatitis often apply many topical products, and a contact allergy to a preservative, fragrance, or medication ingredient can develop on top of existing eczema. When eczema worsens in an unusual pattern or fails to respond to appropriate treatment, patch testing can identify a hidden contact allergen.
No single root, no single fix. Claims that eczema originates entirely in the gut, in one food, or in one environmental toxin, and that removing it will produce a cure, do not match the evidence. Atopic dermatitis grows from several interacting roots, and their relative weight differs from one person to the next.
Treatment targets the roots that can be reached. Daily emollients and gentle skin care support the barrier. Topical anti-inflammatory treatment calms the immune component during and between flares. For moderate to severe disease, targeted therapies that block interleukin-4, interleukin-13, or related pathways act directly on the immune root. Whether early treatment can change the long-term course of the disease is examined in the article on whether eczema can be cured.
Identify triggers, but do not chase them endlessly. Recognizing personal triggers reduces the number and severity of flares. Searching for one hidden cause at the expense of effective treatment usually prolongs inflammation rather than resolving it.
A consultation at the Centre for Medical and Surgical Dermatology allows the contribution of each root to be assessed individually, personal triggers to be identified, 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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