Eczema doesn’t have a single cause. It develops from a combination of genetic vulnerability, an overactive immune system, and environmental triggers that together compromise the skin’s ability to protect itself. Around 9.6% of people worldwide are affected, and the condition runs strongly in families, though the specific mix of causes varies from person to person.
A Faulty Skin Barrier Starts the Problem
Healthy skin works like a brick wall. Skin cells are the bricks, and a mortar of natural oils (called lipids) holds everything together, locking moisture in and keeping irritants out. In eczema, both the bricks and the mortar are defective.
The most significant genetic discovery in eczema research involves a protein called filaggrin. Produced by a gene on chromosome 1, filaggrin plays two critical roles: it helps flatten and compress skin cells into the tight, overlapping layers that form your outermost skin barrier, and when it eventually breaks down, its fragments become part of your skin’s natural moisturizing system. People who carry mutations in the filaggrin gene produce little or no filaggrin protein. The result is a poorly formed outer skin layer that loses water easily and lets allergens, bacteria, and irritants slip through.
The lipid composition of eczema skin is also abnormal. Researchers have found that while the total amount of skin lipids may be similar to healthy skin, the specific types are different. Certain long-chain lipids that are essential for barrier integrity are significantly reduced. The remaining lipids have shorter molecular chains on average, which changes how they organize themselves and creates gaps in the skin’s protective seal. The shorter these lipid chains, the more water escapes through the skin, a measurement that correlates directly with barrier damage.
The Immune System Overreacts
A compromised skin barrier alone doesn’t fully explain eczema. The immune system plays an equally central role. In eczema, a specific branch of the immune response (called the type 2 pathway) is chronically overactivated. Two signaling molecules drive this process. These molecules trigger a cascade: they push immune cells toward an allergic-type response, recruit inflammatory cells like eosinophils and mast cells to the skin, and sustain the inflammation by continuously calling in more immune cells and producing more inflammatory signals.
This same pair of immune signals is also directly responsible for the intense itch that defines eczema. They stimulate itch-sensing nerve fibers through pathways that don’t respond to antihistamines, which is why antihistamines often do little for eczema itch. The scratching that follows damages the skin barrier further, letting in more irritants, which triggers more inflammation. This itch-scratch cycle is one of the main reasons eczema persists and flares.
Genetics Set the Stage
Eczema is strongly hereditary. If one or both parents have eczema, asthma, or hay fever, their children are significantly more likely to develop it. The filaggrin gene mutations are the best-studied genetic risk factor, but they account for only a portion of cases. Many people with eczema have normal filaggrin genes, and not everyone with filaggrin mutations develops the condition. Dozens of other genes involved in immune regulation, skin structure, and inflammation also contribute smaller amounts of risk.
What genetics really do is set a threshold. They determine how resilient your skin barrier is and how easily your immune system tips into an allergic response. Environmental factors then determine whether you cross that threshold.
Skin Bacteria and the Microbiome
Healthy skin hosts a diverse community of bacteria that help keep the barrier functioning and prevent any single species from dominating. In eczema, this diversity collapses. During a flare, the bacterial community on affected skin narrows dramatically, with one species in particular taking over: Staphylococcus aureus.
A meta-analysis of 95 studies found that S. aureus is present on 70% of eczema lesions, compared to roughly 20% of healthy skin. The colonization rate climbs with disease severity. During severe flares, deep genetic sequencing has shown that patients are typically colonized by a single dominant strain. While there’s no proof that S. aureus directly causes eczema, there’s strong evidence it worsens flares. It produces proteins that act as “superantigens,” triggering exaggerated immune responses, and it thrives in the low-diversity, barrier-compromised environment that eczema creates.
Restoring microbial diversity is one reason why some treatments, like dilute bleach baths, can help reduce flare frequency.
Early Life Exposures Shape the Immune System
The rising prevalence of eczema in industrialized countries points to environmental changes over the past several decades. One well-supported explanation is that children in modern, highly sanitized environments encounter fewer of the microbes that train the immune system to tolerate harmless substances. Without that training, the immune system is more likely to overreact to things like pollen, pet dander, or food proteins.
This concept extends to the gut. Early disruptions to intestinal bacteria, whether from antibiotic use in infancy, cesarean delivery, or diet, can interfere with the development of immune tolerance. A healthy gut microbiome promotes the growth of regulatory immune cells that keep allergic responses in check. When that process is disrupted, the immune system leans toward the type 2, allergy-prone state that underlies eczema.
Environmental and Climate Triggers
For someone with the genetic and immune predisposition, a range of environmental factors can trigger or worsen flares. Low humidity and cold temperatures reduce the skin’s ability to hold moisture and make it more vulnerable to mechanical damage. This is why eczema commonly worsens in winter and improves in summer for many people.
Air pollution also plays a measurable role. Outdoor particulate matter contributes to both the onset and severity of eczema symptoms. Indoor pollutants matter too: tobacco smoke can promote atopic disease in genetically susceptible individuals, and volatile organic compounds (from cleaning products, paints, and furnishings) combined with dust mite allergens can compound skin barrier damage in sensitized people. Common personal triggers include fragranced products, rough fabrics like wool, hot water, sweat, and certain detergents.
Stress and the Brain-Skin Connection
The link between stress and eczema flares isn’t just psychological. Your skin has its own stress-response system that mirrors the one in your brain. When you’re under chronic stress, your body produces cortisol, which at elevated levels suppresses certain immune functions while pushing the immune system toward the same type 2, allergy-prone response that drives eczema. Paradoxically, chronic stress can also eventually blunt the cortisol response, leaving inflammatory signals unchecked.
Your skin cells themselves produce stress hormones locally. Skin cells in hair follicles, oil glands, and the outer layer all generate these signals in response to stressors like UV exposure, inflammation, or immune activation. This means the skin can amplify its own stress response independently of what’s happening in the brain, creating a local feedback loop where stress-driven inflammation damages the barrier, which triggers more inflammation.
How a Broken Barrier Leads to Other Allergies
One of the most important implications of eczema’s causes is what happens next. When the skin barrier is compromised, allergens like food proteins and dust mite particles can penetrate the skin and encounter immune cells in the outer layers. These immune cells capture the allergens, travel to lymph nodes, and train the broader immune system to react to those substances. This is how a skin problem can lead to food allergies, hay fever, and asthma, a progression doctors call the atopic march.
Research in both humans and animal models shows that this process is real and measurable. Children with early-onset eczema and filaggrin mutations who become sensitized to foods are at increased risk not only for persistent eczema but also for developing asthma later. Skin prick tests showing dust mite sensitivity in one- and two-year-olds predicted wheezing at age 12. There’s also evidence that peanut allergy may develop through skin exposure to peanut proteins in household dust, rather than through eating peanuts, which supports the idea that the damaged skin barrier is the entry point for allergic sensitization. This understanding has shifted how researchers think about preventing allergies: protecting and repairing the skin barrier early may help reduce the risk of developing these related conditions.