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 every case. Instead, the condition develops when inherited vulnerabilities in skin structure and immune function collide with outside triggers like bacteria, allergens, and pollutants.
A Weakened Skin Barrier Starts the Problem
Healthy skin works like a brick wall: tough protein cells held together by a mortar of fats called ceramides. In atopic dermatitis, both the bricks and the mortar are compromised. The most well-studied defect involves a protein called filaggrin, which helps skin cells flatten and lock together to form a tight outer layer. Mutations in the gene that produces filaggrin are found in roughly 16% of people with atopic dermatitis overall, with higher rates in white populations (about 28%) and lower rates in Black populations (about 6%). When filaggrin is missing or reduced, the skin loses moisture more easily and lets irritants, allergens, and bacteria slip through.
The fat composition of the skin is also altered. People with atopic dermatitis show abnormal levels of certain ceramides, the lipids that normally seal the gaps between skin cells. Without the right mix of these fats, the barrier becomes porous. Water escapes, the skin dries out, and the stage is set for inflammation.
The Immune System Overreacts
Once the barrier is compromised, the immune system responds in a way that makes things worse rather than better. In atopic dermatitis, a specific branch of the immune system (called the type 2 response) becomes dominant. This branch produces signaling molecules that were designed to fight parasites, but in atopic dermatitis they fire inappropriately and cause widespread skin inflammation.
Two of these signaling molecules, IL-4 and IL-13, do particular damage. They reduce the skin’s production of filaggrin and other barrier proteins, which means the immune response actively worsens the barrier defect that triggered it in the first place. They also suppress the skin’s natural antimicrobial defenses, leaving it more vulnerable to infection. And they act directly on nerve endings in the skin, contributing to the intense itch that defines the condition.
A third molecule, IL-31, is considered a primary driver of itch in atopic dermatitis. Produced mainly by overactive immune cells, IL-31 stimulates nerve receptors in the skin and triggers the urge to scratch. It’s even present in sweat, which helps explain why sweating often makes the itch worse.
The Itch-Scratch Cycle Fuels Itself
Itching in atopic dermatitis is not the same as a mosquito bite itch. Most everyday itching responds to antihistamines because it travels through histamine-driven nerve pathways. The chronic itch of atopic dermatitis primarily uses a different nerve pathway that doesn’t respond well to antihistamines. This pathway is activated by immune molecules, enzymes, and other chemical signals from inflamed skin.
When you scratch, nerve fibers release compounds like substance P, which triggers what’s called neurogenic inflammation: blood vessels dilate, fluid leaks into surrounding tissue, and immune cells called mast cells dump their contents into the skin. This creates more inflammation, which creates more itch, which leads to more scratching. Over time, repeatedly scratched skin thickens and becomes leathery, a change driven partly by IL-13 promoting collagen buildup in the tissue.
Bacteria Colonize Damaged Skin
One of the most significant contributors to flares is a bacterium called Staphylococcus aureus. While it lives on the skin of 20 to 30% of healthy people (usually in the nose), it colonizes the damaged skin of 70 to 90% of people with atopic dermatitis. The colonization rate climbs with age: about 50% of infants with active flares carry it, compared to 80% of children and nearly 88% of adults.
S. aureus doesn’t just sit on the skin passively. It produces enzymes that break down barrier proteins, making the skin even more permeable. It releases toxins that act as superantigens, molecules that provoke an exaggerated immune response far out of proportion to the actual threat. It also forms biofilms, sticky protective layers that shield the bacteria from both the immune system and topical treatments, making colonization persistent and difficult to clear. As S. aureus takes over, it crowds out the diverse community of beneficial microbes that normally keep the skin healthy, reducing overall microbial diversity and further weakening defenses.
Genetics Set the Stage
Atopic dermatitis runs in families, and the risk is highest when both parents have a history of allergic conditions like eczema, asthma, or hay fever. The filaggrin gene mutations described above are the strongest single genetic risk factor, but they explain only a fraction of cases. Many people with atopic dermatitis have no filaggrin mutation at all, and some people who carry the mutation never develop eczema. Dozens of other genes involved in immune regulation, skin barrier maintenance, and inflammatory signaling contribute smaller amounts of risk. The genetics load the gun, but environmental factors pull the trigger.
Environmental Triggers and Early Life Exposure
A range of environmental exposures can provoke or worsen atopic dermatitis. Air pollutants including tobacco smoke, volatile organic compounds (such as formaldehyde and toluene), nitrogen dioxide, and particulate matter all act as risk factors. These pollutants appear to induce oxidative stress in the skin, damaging barrier function and provoking immune reactions. Dry air, harsh soaps, wool clothing, and temperature extremes are common everyday triggers that further compromise an already fragile barrier.
There’s also evidence that the broader microbial environment during early childhood plays a role in whether atopic dermatitis develops at all. The hygiene hypothesis, first proposed in 1989, suggests that children exposed to a wider variety of microbes early in life are less likely to develop allergic conditions. The idea is that diverse microbial exposure trains the immune system to tolerate harmless substances rather than overreacting to them. Children raised with older siblings, on farms, or with pets tend to have lower rates of atopic dermatitis, supporting the notion that early immune “education” matters.
The Connection to Food Allergies
Atopic dermatitis affects nearly 20% of children, and about 30% of those children also have food allergies. The relationship between the two conditions is complex and often misunderstood. A damaged skin barrier may actually allow food proteins to enter through the skin and sensitize the immune system, meaning the eczema can come first and the food allergy can follow. This is different from the common assumption that eating certain foods causes eczema flares.
Researchers at the NIH have identified a unique subtype of eczema that appears specifically linked to food allergy, suggesting that not all atopic dermatitis behaves the same way. For some children, food allergens genuinely worsen their skin. For others, the two conditions coexist without one directly driving the other. Blanket elimination diets without confirmed food allergy testing are generally not helpful and can lead to nutritional problems, especially in young children.
How These Causes Work Together
What makes atopic dermatitis so persistent is that its causes reinforce each other in a vicious cycle. A genetic barrier defect lets irritants and microbes into the skin. The immune system overreacts with type 2 inflammation, which further degrades the barrier, suppresses antimicrobial defenses, and activates itch nerves. Scratching physically damages the skin, releasing more inflammatory signals and opening the door to S. aureus colonization. The bacteria trigger more immune activation, more barrier damage, and more itch. Environmental pollutants and allergens pour through the compromised barrier, adding fuel at every stage.
This is why effective management typically needs to address multiple causes simultaneously: restoring the barrier with moisturizers, calming the immune overreaction with targeted treatments, reducing bacterial burden, and identifying and minimizing individual environmental triggers. No single intervention breaks every link in the chain, but understanding what drives the cycle makes it possible to interrupt it at several points.