Atopic dermatitis results from a combination of genetic, immune, and environmental factors that together weaken the skin’s protective barrier and trigger chronic inflammation. No single cause explains the condition. Instead, inherited skin barrier defects, an overactive immune response, shifts in skin bacteria, and environmental irritants all interact to produce the itchy, inflamed skin that affects up to 20% of children and roughly 10% of adults worldwide. It ranks as the number one skin disease globally when measured by overall health burden.
Genetic Skin Barrier Defects
The outermost layer of your skin acts like a brick wall: skin cells are the bricks, and a mix of fats (lipids) serves as the mortar holding everything together. In atopic dermatitis, this wall has gaps. The most well-studied genetic cause involves mutations in the gene that produces filaggrin, a protein essential for building and maintaining that outer barrier. Without enough functional filaggrin, the skin loses moisture more easily and lets irritants, allergens, and bacteria slip through.
About 16% of people with atopic dermatitis carry at least one filaggrin gene mutation, though the rate varies by ethnicity. In one large US cohort, 27.5% of white participants carried the mutation compared to 5.8% of African American participants. That means filaggrin mutations are a significant risk factor but far from the whole story. Many people with atopic dermatitis have normal filaggrin genes, and some people with the mutation never develop eczema.
An Immune System Tilted Toward Inflammation
Even without a genetic barrier defect, the immune system itself can drive the disease. In atopic dermatitis, certain immune cells produce excessive amounts of inflammatory signaling molecules. Two of the most important are IL-4 and IL-13. IL-4 pushes the immune system toward allergic-type responses by steering white blood cells into a pattern that produces the antibody IgE, the same antibody behind hay fever, food allergies, and asthma. IL-13, meanwhile, acts directly on skin cells, remodeling tissue and further disrupting barrier function.
This creates a vicious cycle. A weakened barrier lets allergens and irritants penetrate the skin, which activates the immune system, which produces inflammation that damages the barrier even more. The immune response also alters the composition of fats in the skin’s outer layer. Specifically, it reduces the production of long-chain ceramides (the fats most critical for barrier integrity) and increases short-chain versions that are less effective at sealing moisture in. Enzymes responsible for building those longer fat molecules are downregulated in affected skin, meaning the immune dysfunction actively prevents the barrier from repairing itself.
Bacterial Imbalance on the Skin
Healthy skin hosts a diverse community of microorganisms that help keep harmful bacteria in check. In atopic dermatitis, that diversity collapses. Between 70% and 90% of people with the condition are colonized by Staphylococcus aureus on their affected skin, compared to just 20% to 30% of healthy people who carry the bacterium (typically only in the nose).
S. aureus doesn’t just take advantage of damaged skin. It actively makes things worse. The bacterium produces molecules called superantigens that provoke a disproportionate immune reaction, amplifying inflammation far beyond what a normal infection would cause. It also secretes enzymes, including proteases and lipases, that break down skin barrier proteins and fats, increasing permeability and making it easier for allergens and other pathogens to enter. Flares of atopic dermatitis often coincide with spikes in S. aureus populations on the skin.
Environmental Triggers
Genetics and immune dysfunction set the stage, but environmental exposures often determine when and how severely symptoms appear. Air pollution is one of the better-studied triggers. A 1-part-per-billion increase in volatile organic compound (VOC) concentration has been linked to a roughly 26% increase in symptoms the following day. Particulate matter shows a similar pattern: each 1 microgram-per-cubic-meter rise in PM10 corresponds to a 0.44% increase in symptom severity. Ultrafine particles smaller than 0.1 micrometers are particularly associated with increased itching.
Specific pollutants flagged as risk factors include tobacco smoke, formaldehyde, toluene, nitrogen dioxide, and particulate matter. Beyond air quality, low humidity, cold weather, hard water, and contact with irritants like soaps, fragrances, and certain fabrics can all trigger or worsen flares. These exposures don’t cause atopic dermatitis on their own, but they can push already-vulnerable skin past the threshold into active inflammation.
Early Childhood Microbial Exposure
The hygiene hypothesis offers one explanation for why atopic dermatitis has become more common in industrialized countries. The core idea is that reduced exposure to diverse microbes in early life skews immune development. Without enough microbial stimulation, the immune system fails to develop the regulatory responses that normally keep allergic reactions in check. Instead, it tilts toward the allergic, inflammation-heavy pattern seen in atopic dermatitis.
Protective microbes in the gut and on the skin appear to train the immune system to tolerate harmless environmental substances like pollen, pet dander, and dust. Children raised on farms, in larger families, or with early daycare exposure tend to have lower rates of atopic conditions, likely because their immune systems encounter a wider range of microorganisms during the critical window of immune development.
The Itch-Scratch Cycle
Chronic itch is the hallmark symptom of atopic dermatitis, and it has its own biological causes that go beyond simple skin irritation. The nerve fibers in affected skin become hypersensitive. In people with atopic dermatitis, the slow-conducting nerve fibers responsible for transmitting itch signals show high levels of spontaneous firing, meaning they send itch signals to the brain even without an obvious trigger.
This sensitization happens at multiple levels. In the skin itself, a signaling molecule called substance P is overproduced by nerve fibers, causing localized inflammation and amplifying itch. At the spinal cord level, substance P binds to receptors that keep itch signals flowing to the brain. Over time, the nervous system essentially turns up the volume on itch, so that stimuli like light touch or warmth, things that wouldn’t normally itch, begin to trigger intense scratching urges. Scratching then damages the barrier further, introduces bacteria, and provokes more inflammation, restarting the entire cycle.
Connection to Other Allergic Conditions
Atopic dermatitis is often the first condition to appear in a sequence sometimes called the “atopic march,” where eczema in infancy is followed by food allergies, asthma, and hay fever later in childhood. In one large adolescent study, 36% of people with a history of atopic dermatitis reported having had asthma, and 61% reported allergic rhinitis at some point. The shared underlying cause is the same immune imbalance: excessive IL-4 and IgE production that manifests in different organs at different ages.
That said, this progression is not inevitable. Roughly half of children with atopic dermatitis never go on to develop asthma or hay fever. Some researchers now view these conditions less as a predictable march and more as a cluster of related diseases that share common immune and genetic roots but follow different paths depending on the individual.