Eczema develops through a combination of genetic predisposition, immune system overactivity, and environmental triggers. It isn’t something you “catch” from another person or from a single exposure. Instead, it results from an underlying defect in how your skin protects itself, combined with an immune response that overreacts to irritants and allergens that wouldn’t bother most people. About 9.6% of the global population has eczema, and 80% of cases begin before age 6, though one in four adults report their symptoms started in adulthood.
It Starts With a Faulty Skin Barrier
Healthy skin works like a brick wall. Tough, flat cells form the “bricks,” and a mixture of natural fats fills the gaps between them like “mortar.” About 50% of that mortar is made up of ceramides, with cholesterol and fatty acids making up the rest. This seal keeps moisture in and keeps irritants, allergens, and bacteria out.
In people with eczema, all three of those key fats are reduced, especially ceramides. The mortar becomes leaky. Water escapes from the skin faster than normal (a process called transepidermal water loss), which is why eczema-prone skin feels dry even in areas without a visible rash. At the same time, the weakened barrier lets irritants and allergens slip through more easily, setting off inflammation underneath.
This “outside-in” theory views the barrier defect as the first domino. Once the barrier fails, irritants penetrate, the immune system reacts, and the cycle of itching and inflammation begins.
The Role of Genetics
The single most studied genetic factor in eczema is a mutation in the gene that produces a protein called filaggrin. This protein does several critical jobs: it helps flatten and strengthen the outermost skin cells, it breaks down into molecules that form the skin’s natural moisturizing factor, and it helps maintain the slightly acidic pH that keeps the barrier functioning properly. When the gene is mutated, filaggrin production drops, and the skin loses structural integrity, hydration, and its ability to fend off invaders.
Between 20% and 30% of people with eczema carry a filaggrin gene mutation, compared to 8% to 10% of the general population. That means the mutation significantly raises your risk, but it’s not required. The majority of people with eczema have normal filaggrin genes, which means other genetic and environmental factors are also at play. A family history of eczema, asthma, or hay fever is one of the strongest predictors. If one or both of your parents had any of these conditions, your odds of developing eczema go up substantially.
An Overactive Immune Response
Once irritants or allergens breach the weakened skin barrier, the immune system mounts a response that’s disproportionate to the threat. In eczema, a specific branch of the immune system dominates. Certain signaling molecules, particularly IL-4 and IL-13, drive the skin into a state of chronic inflammation. They make the outer skin layer thicken abnormally, worsen the barrier defect, and create the redness and swelling you can see on the surface.
Other immune signals pile on. IL-22 appears to play a particularly important role in acute flares, amplifying the damage that IL-4 and IL-13 initiate. The result is a vicious cycle: barrier damage triggers immune activation, and immune activation further damages the barrier. This is why eczema tends to flare and remit rather than stay constant. Any new trigger restarts the loop.
Skin Bacteria and the Microbiome
Your skin is home to a diverse community of bacteria, most of which are harmless or even beneficial. In eczema, that community becomes unbalanced. One species in particular, Staphylococcus aureus, colonizes the skin of virtually all people with eczema. In healthy skin, S. aureus is kept in check by competing bacteria. When the barrier is damaged and the immune environment shifts, S. aureus thrives and actively drives further inflammation.
Research has shown that targeting these bacterial imbalances with specific antibiotics can nearly eliminate skin inflammation in affected areas, confirming that the bacteria aren’t just bystanders. They’re active participants in flares. This is one reason why eczema-prone skin is more susceptible to skin infections and why flares sometimes worsen suddenly without an obvious new trigger.
Environmental Triggers That Start or Worsen Flares
Genetics and immune dysfunction set the stage, but environmental factors pull the trigger. These don’t cause eczema on their own, but they provoke flares in people whose skin is already vulnerable. The major categories include:
- Irritants: soap, harsh laundry detergents, solvents, antiseptics, chlorine in swimming pools, cosmetics, and perfumes. Even some prescription and over-the-counter skin creams can irritate eczema-prone skin.
- Fabrics: wool, synthetic fibers, and clothing with rough seams directly against the skin.
- Climate and weather: cold, dry winter air, low humidity, and high ambient temperatures. Most people with eczema get itchier and redder when they’re hot. Washing in hard water, which raises the skin’s pH, and bathing in very hot water also strip the barrier.
- Allergens: dust mites, pollen, grass, cat dander, and certain microbes.
- Early life exposures: exposure to particulate pollution during pregnancy and the first 30 weeks of life may increase the risk of a child developing eczema.
Cold, damp climates can make eczema more resistant to treatment over time. Emotional stress is also recognized as a trigger, though it works through indirect pathways by ramping up immune activity and increasing behaviors like scratching.
Why Some People Develop It Later in Life
Although eczema is strongly associated with childhood, adult-onset cases are not rare. About one in four adults with eczema report that their symptoms first appeared in adulthood. Global studies show adult prevalence ranges widely, from 3% to 10%, depending on the population studied.
Adult-onset eczema can be triggered by a new environmental exposure, a shift in climate, occupational contact with irritants (healthcare workers who wash their hands frequently, for example), or hormonal changes. Some adults may have carried a genetic susceptibility their entire lives without meeting the right combination of triggers until later. The underlying mechanism is the same: barrier dysfunction meets immune overreaction, regardless of when it first shows up.
How Eczema Is Diagnosed
There’s no single blood test or biopsy that confirms eczema. Doctors diagnose it based on a combination of what they see and what you report. The most widely used framework requires at least three of four major features: persistent itching, a rash in characteristic locations (the creases of elbows and knees in adults, the face and outer limbs in infants), a pattern of flaring and improving over time, and a personal or family history of eczema, asthma, or hay fever.
Beyond those, doctors look for supporting signs like unusually dry skin, darkening under the eyes, extra creases on the palms, small bumps around hair follicles on the upper arms, and a history of skin infections. The location and pattern of the rash matter more than any single test, which is why a thorough skin exam and conversation about your history are the core of diagnosis.