Allergies happen because your immune system misidentifies a harmless substance, like pollen or peanut protein, as a dangerous invader and mounts a full defensive attack against it. Roughly 3 in 10 U.S. adults and children reported having at least one allergy in 2024, whether seasonal, food-related, or skin-based. The tendency to develop allergies comes down to a combination of your genes, your early childhood microbial exposure, and the specific physical properties of the proteins your body encounters.
What Happens Inside Your Body
An allergic reaction is a two-stage process: sensitization and triggering. The first time you encounter an allergen, say cat dander or tree pollen, your immune system may decide it’s a threat. Certain immune cells release signaling molecules that tell your B cells (the antibody factories of your immune system) to produce a specific type of antibody called IgE. These IgE antibodies don’t float around freely for long. They latch onto mast cells and basophils, two types of immune cells packed with inflammatory chemicals, and wait there like loaded mousetraps.
The second time you encounter that same allergen, it binds to the IgE antibodies already sitting on your mast cells. This triggers those cells to burst open in a process called degranulation, dumping histamine and a cocktail of other inflammatory compounds into surrounding tissue. Histamine is the main culprit behind most allergy symptoms you recognize. It causes blood vessels to widen and leak fluid, which produces swelling, redness, and that runny nose. It makes smooth muscle in your airways contract, leading to wheezing and tightness in the chest. It triggers nerve endings, causing itching and sneezing.
Where the reaction hits depends on where the allergen lands. Pollen in your nose triggers sneezing and congestion. A food allergen in your gut can cause cramping and vomiting. In severe cases, the reaction goes systemic, affecting multiple organ systems at once, which is what happens during anaphylaxis.
Why Some People Get Allergies and Others Don’t
Genetics play a major role. Researchers have identified at least 62 genes associated with allergic conditions, many of them related to immune function and the structural integrity of your skin and mucous membranes. One of the most studied is the filaggrin gene (FLG), which helps build the outermost protective barrier of your skin. People with loss-of-function mutations in this gene have a nearly twofold increased risk of developing asthma and hay fever by age 10. When your skin barrier is compromised, allergens can penetrate more easily, giving the immune system more opportunities to react to things it shouldn’t.
But genes alone don’t explain the allergy epidemic. Allergic diseases have surged in developed countries over the past few decades, far too quickly to be driven by genetic changes. Something in our environment shifted.
The Role of Early Microbial Exposure
Your immune system at birth is like a computer with software installed but almost no data loaded. During the first years of life, it needs exposure to a wide range of microorganisms, from bacteria on other people’s skin to microbes in soil and the natural environment, to learn what’s actually dangerous and what can be safely ignored. If those inputs are inadequate, the immune system’s regulatory mechanisms can fail. It starts attacking not just harmful pathogens but also harmless targets like pollen, dust mites, and food proteins.
This concept was originally called the “hygiene hypothesis,” but researchers now consider that term misleading. It’s not that modern hygiene or handwashing causes allergies. The real issue is a loss of diverse microbial contact, particularly with the symbiotic organisms humans co-evolved with over millennia, sometimes called “old friends.” Children raised on farms, around animals, or with greater exposure to diverse environmental microbes consistently show lower rates of allergic disease. The diversity of microbial exposure matters most: a broad range of harmless bacteria during infancy builds a larger library of organisms the immune system learns to tolerate, which helps prevent it from overreacting to novel but innocent substances later.
Preventive efforts, researchers emphasize, need to focus on early life. Once the immune system’s regulatory patterns are set, they’re harder to recalibrate.
What Makes a Protein Allergenic
Not every protein you encounter triggers an allergic response, even in people prone to allergies. The proteins that do tend to share a specific set of physical characteristics. They’re small, typically in the range of 14 to 17 kilodaltons (a unit of molecular weight). They’re unusually stable, resistant to heat and to the digestive enzymes in your stomach and intestines. Many are held together by internal chemical bonds called disulfide bridges that give them a compact, tightly folded three-dimensional shape, making them hard for your body to break apart.
This durability is key. Most proteins you eat get shredded into unrecognizable fragments long before they reach the parts of your gut where immune cells are waiting. Allergenic proteins survive that journey intact, arriving in a form the immune system can detect and, in susceptible people, misidentify as a threat. Many of these proteins also have the ability to interact with cell membranes, which may help them cross the barriers of your gut lining, skin, or airways more easily.
Environmental Factors That Make It Worse
Air pollution doesn’t cause allergies on its own, but it significantly worsens them. Particulate matter, nitrogen dioxide, and ozone all inflame the airways in ways that prime them for stronger allergic reactions. Studies in Mexico City found that concentrations of fine particulate matter and ozone were significantly linked to increased asthma attacks in children. Diesel exhaust particles are particularly problematic. Research has shown that certain genetic variants in oxidative stress pathways make some people especially vulnerable to diesel exhaust worsening their allergic rhinitis, a clear example of genes and environment working together.
Ozone exposure alone has been tied to more frequent hospital admissions, worsening symptoms, and greater need for rescue medication among people with existing allergies and asthma.
Allergy vs. Intolerance
These terms get used interchangeably, but they describe fundamentally different processes. A food allergy is an immune system reaction to a specific protein. It involves IgE antibodies, mast cell activation, and histamine release. Reactions are often rapid and can be life-threatening. A food intolerance, like lactose intolerance, has nothing to do with the immune system. It means your body lacks the ability to properly digest a particular food component, leading to gastrointestinal discomfort like bloating, gas, or diarrhea. Intolerances are uncomfortable but not dangerous in the way allergies can be.
How Allergies Are Being Treated
Beyond avoidance and antihistamines, one of the most promising developments is oral immunotherapy, where patients consume tiny, gradually increasing amounts of their allergen under medical supervision to retrain the immune system’s response. Results for peanut allergy have been encouraging: in one trial of 270 children and teens, 79% achieved desensitization to a significant dose of peanut protein. Among preschool-aged children, who tend to respond better, success rates reached nearly 87% for achieving their maintenance dose, and over 98% could tolerate a meaningful amount of peanut protein afterward.
Age matters. Younger children generally respond better and may be more likely to achieve lasting tolerance, meaning they can eventually eat the food without ongoing treatment. For older patients, desensitization often requires continued daily exposure to maintain protection. The field is still working out optimal dosing, timing, and which patients benefit most, but for many families dealing with severe food allergies, it represents a real shift from pure avoidance to active management.