Allergies develop when your immune system mistakenly treats a harmless substance, like pollen or peanut protein, as a dangerous invader. This isn’t a one-step process. Your body first has to encounter the substance and build a targeted defense against it, a phase called sensitization. Only on a later exposure does the actual allergic reaction occur. What determines whether you become sensitized in the first place comes down to a mix of genetics, how your immune system was trained in early life, and sometimes even the route through which you first encountered the substance.
How Your Body Builds an Allergy
Allergic reactions happen in two distinct stages, and understanding both explains why you don’t react the very first time you encounter an allergen.
During the first stage, sensitization, your immune cells encounter a protein from something like dust mites, tree pollen, or a food. Specialized cells process that protein and present it to your immune system’s T cells. In people prone to allergies, these T cells take a specific developmental path: they become a subtype that drives allergic inflammation. These T cells then instruct B cells (the antibody factories of your immune system) to produce a special class of antibody called IgE, which is specifically shaped to recognize that one allergen. The freshly made IgE antibodies circulate through your body and attach themselves to the surface of mast cells, which are packed with inflammatory chemicals and stationed throughout your skin, airways, and gut lining. This entire setup process takes days to weeks, and you feel nothing while it happens.
The second stage is the reaction itself. When you encounter the same substance again, its proteins latch onto the IgE antibodies already sitting on your mast cells. This cross-linking acts like a trigger, causing the mast cells to burst open and dump their contents: histamine, prostaglandins, leukotrienes, and a cascade of other inflammatory signals. Histamine dilates blood vessels and makes them leaky, which causes swelling, redness, and itching. Leukotrienes tighten the muscles around your airways, making it harder to breathe. This chemical flood can produce hives, a runny nose, stomach cramps, or, in severe cases, a body-wide reaction that drops blood pressure and compromises breathing within minutes.
Why Some People Develop Allergies and Others Don’t
Genetics plays the largest known role. If one parent has allergies, a child is 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 an allergy to a specific substance but rather a tendency for the immune system to overproduce IgE and favor the inflammatory T cell pathway.
One well-studied genetic factor involves mutations in the gene that produces filaggrin, a protein critical to your skin’s barrier function. Filaggrin helps keep skin hydrated, maintains its slightly acidic pH, and forms a tight seal against the outside world. About 8 to 10% of the general population carries a mutation that reduces filaggrin production. When the skin barrier is compromised, allergens can slip through more easily and encounter immune cells beneath the surface, kickstarting sensitization. This is one reason eczema in infancy is strongly linked to later food allergies and asthma, a progression researchers call the “atopic march.” A child’s broken skin barrier allows food proteins or airborne allergens to penetrate and prime the immune system for allergic responses that can eventually affect the airways.
How Early Life Shapes Allergy Risk
Your immune system doesn’t come pre-programmed to tolerate harmless proteins. It has to learn, and most of that learning happens in the first months and years of life. A major teacher is the community of bacteria colonizing your gut and respiratory tract. Birth cohort studies in humans and experiments in mice point to the same conclusion: children who lack diverse microbial colonization early on are more susceptible to allergic sensitization. Specific groups of gut bacteria, particularly certain species of Clostridia and Bacteroides, promote the development of regulatory T cells, the immune cells responsible for teaching the rest of the system to stand down when a substance is harmless.
In animal studies, introducing these bacteria into the gut reversed the immune imbalances associated with allergy, but only when done early in life. This aligns with the broader observation that children raised on farms, in larger families, or with pets tend to have lower allergy rates. Their immune systems encounter a richer microbial environment during the critical training window. Conversely, factors that reduce microbial diversity, like frequent antibiotic use in infancy or highly sanitized environments, may leave the immune system more likely to default toward allergic responses.
This understanding has also reshaped how pediatricians approach food allergies. The National Institute of Allergy and Infectious Diseases now recommends introducing peanut-containing foods to high-risk infants (those with severe eczema or egg allergy) as early as 4 to 6 months of age. Feeding the immune system these proteins through the gut, where tolerance is more easily established, appears to reduce the chance of developing a peanut allergy compared to avoidance.
Cross-Reactivity: Allergies That Spread
Sometimes an allergy to one substance triggers reactions to seemingly unrelated things. This happens because the IgE your body made against one protein can also recognize a structurally similar protein from a different source. The most common example is pollen-food allergy syndrome. If you’re allergic to birch pollen, you may notice tingling or itching in your mouth when eating raw apples, cherries, or carrots, because those foods contain proteins that share a similar shape with birch pollen proteins. Similarly, olive pollen allergy can cross-react with peach, where the relevant proteins share roughly 50 to 57% structural similarity in their key regions, enough for the same IgE antibodies to bind both.
These cross-reactions typically cause mild, localized symptoms (itching or swelling in the mouth and throat) because the food proteins are fragile and break down quickly in the stomach. Cooking the food usually eliminates the reaction entirely, since heat destroys the protein’s shape.
Allergy Versus Intolerance
Not every bad reaction to a food or substance is an allergy. A true allergy involves the immune system producing IgE against a specific protein, which means it carries the potential for severe, rapid, whole-body reactions. A food intolerance, by contrast, typically involves the digestive system alone. Lactose intolerance, for example, happens because your body doesn’t produce enough of the enzyme needed to break down milk sugar. The result is bloating, gas, or diarrhea, but not hives, throat swelling, or anaphylaxis. The distinction matters because allergies can escalate unpredictably, while intolerances, though uncomfortable, don’t pose the same acute danger.
When Allergies Become Dangerous
Most allergic reactions stay localized: sneezing from pollen, hives from a food, itchy eyes from pet dander. Anaphylaxis occurs when the reaction goes systemic, affecting two or more organ systems at once. The clinical markers include skin symptoms (widespread hives, flushing, or swelling) combined with respiratory distress (wheezing, throat tightness, difficulty breathing), a sudden drop in blood pressure (dizziness, fainting), or severe gastrointestinal symptoms (intense cramping, repeated vomiting). These reactions develop within minutes to hours of exposure and can be fatal without treatment.
What determines whether someone has a mild reaction or a life-threatening one isn’t fully predictable. The amount of allergen, how it entered the body, and the individual’s current state (exercise, alcohol use, and concurrent illness can all amplify reactions) play roles. A person who previously had only mild symptoms can experience anaphylaxis on a subsequent exposure, which is why allergy diagnosis and preparedness matter even when past reactions seemed minor.