Allergies develop when your immune system mistakenly identifies a harmless substance, like pollen, dust, or a food protein, as a threat and mounts a defensive response against it. About one in three American adults has at least one diagnosed allergic condition, whether seasonal allergies, eczema, or a food allergy. The process isn’t random. It involves a specific chain of biological events, shaped by your genetics, your environment, and the timing of your exposures.
What Happens Inside Your Body
Allergies don’t appear the first time you encounter a substance. They require at least two exposures, because the first encounter is when your immune system quietly prepares its overreaction.
During that initial exposure, your immune cells detect the allergen (inhaled, swallowed, or absorbed through the skin) and decide it’s dangerous. Specialized immune cells called B-cells produce antibodies designed to target that specific substance. Think of these antibodies as “WANTED” posters that spread throughout your body, telling other immune cells exactly what to look for. Those antibodies attach to mast cells, which are stationed in your skin, lungs, gut, and nasal passages. At this point, you feel nothing. But your immune system is now primed.
On the next exposure, the mast cells recognize the allergen immediately. They release histamine and other inflammatory chemicals, triggering the symptoms you associate with allergies: sneezing, itching, swelling, hives, or in severe cases, anaphylaxis. Each antibody is highly specific, which is why you can be allergic to birch pollen but not grass pollen, or to peanuts but not almonds. With repeated exposures, the severity of the reaction can increase over time.
Why Some People Develop Allergies and Others Don’t
Genetics plays a major role. If one parent has allergies, their children are 30 to 50% more likely to develop them. If both parents have allergies, that risk jumps to 60 to 80%. What’s inherited isn’t a specific allergy (your mother’s ragweed allergy won’t necessarily become yours) but rather a tendency for the immune system to overreact to harmless substances.
But genetics alone doesn’t explain everything. Identical twins don’t always share the same allergies. The environment you grow up in, particularly in your first few years of life, appears to shape whether your genetic predisposition actually leads to allergic disease.
The Role of Early Microbial Exposure
One of the most consistent findings in allergy research is that children raised in microbe-rich environments develop fewer allergies. This concept, sometimes called the hygiene hypothesis, proposes that early-life exposure to bacteria, fungi, and other microorganisms trains the immune system to distinguish real threats from harmless ones.
The evidence is striking. A study comparing Amish farming children in Indiana to Swiss children found dramatic differences. Hay fever affected just 0.6% of Amish children, compared to 11.6% of Swiss non-farm children. Asthma rates were 5.2% versus 11.2%. Only 7.2% of Amish children showed sensitivity to airborne allergens on testing, compared to 44.2% of non-farm Swiss children. Amish families live in close daily contact with livestock, soil, and barn dust, providing constant low-level microbial exposure from birth.
Researchers have pinpointed some of the specific mechanisms at work. Bacterial markers found in household dust during infancy were associated with lower rates of allergic sensitization and asthma by the time children reached school age. In a nationally representative U.S. study, higher levels of these bacterial markers in house dust were linked to lower risk of developing allergies to pets and pollens in children. Interestingly, the same exposure in adults was associated with higher allergy risk, suggesting there’s a critical window in early childhood when microbial exposure has its protective effect.
Your Body’s Physical Barriers Matter Too
Your skin, lungs, and gut lining act as physical walls that keep foreign substances out of deeper tissue. When these barriers are intact, allergens sit on the surface and get cleared away harmlessly. When they’re compromised, allergens slip through the gaps and encounter immune cells that can trigger the sensitization process.
Damaged barriers cause surface cells to release alarm signals that push the immune system toward the type of response associated with allergies. This creates a feedback loop: the resulting inflammation further weakens the barrier, allowing even more allergens through. It’s one reason eczema in infancy, which disrupts the skin barrier, is a strong predictor of later food allergies and asthma. Substances that damage these barriers, whether from pollution, harsh chemicals, or certain infections, can increase allergy risk by giving allergens access to immune cells they wouldn’t normally encounter.
Developing New Allergies as an Adult
Allergies aren’t limited to childhood. New allergies can appear at any age, and roughly a quarter of U.S. adults currently have a diagnosed seasonal allergy. Food allergies affect 6.7% of adults, and some of those developed well after childhood.
Adult-onset allergies remain somewhat unpredictable, but one pattern stands out: infrequent exposure to a potential allergen may actually increase the risk. Shellfish allergy, for example, commonly appears in adulthood, potentially because most people eat shellfish only a few times a year. Moving to a new region with unfamiliar pollen can also trigger new seasonal allergies, since your immune system encounters proteins it hasn’t been exposed to before.
Cross-reactivity is another route. If you’re allergic to certain pollens, your immune system can start reacting to proteins in raw fruits, vegetables, or tree nuts that look structurally similar. This is called pollen food allergy syndrome. Your body mistakes the food protein for the pollen it already recognizes, causing itching or tingling in the mouth. It’s a common way for people who’ve had hay fever for years to suddenly seem “allergic” to apples or celery.
Early Food Introduction and Prevention
For parents trying to reduce their child’s allergy risk, the guidance has shifted significantly in recent years. Delaying the introduction of common allergens like peanut, egg, dairy, and sesame does not prevent allergies. In fact, there’s evidence it increases the chance of developing them.
Current recommendations encourage introducing small amounts of allergenic foods once a baby is ready for solids, typically around six months. For babies at high risk of peanut allergy (those with severe eczema or an existing egg allergy), peanut-containing products should be introduced as early as four to six months, ideally after discussion with a pediatrician about the safest approach. Start with small tastes, and if there’s no reaction, gradually increase the amount and keep the food in the diet regularly. Practical portions look like about two teaspoons of peanut butter or a third of a well-cooked egg. Whole nuts are a choking hazard for young children and should be avoided regardless of allergy status.
The goal is to expose the immune system to these proteins early and consistently, during the window when it’s still learning what’s safe and what isn’t. Regular exposure appears to teach tolerance, while sporadic or delayed exposure may do the opposite.