Allergies are caused by your immune system treating a harmless substance, like pollen or a food protein, as if it were a dangerous invader. Between 10% and 30% of the world’s population deals with allergic rhinitis alone, and rates of food allergies and other allergic conditions have climbed steadily in recent decades. The underlying problem isn’t the substance itself. It’s a misfiring of the body’s defense system.
How Your Immune System Creates an Allergy
An allergy develops in two stages. The first is silent. During an initial exposure to an allergen (say, peanut protein or cat dander), your immune system decides, incorrectly, that it’s a threat. White blood cells called B cells produce a specific type of antibody designed to recognize that particular substance. These antibodies then attach to mast cells, which are stationed throughout your skin, airways, and gut, essentially arming them like tiny land mines.
Nothing happens yet. You don’t sneeze, itch, or swell. But your body is now “sensitized.”
The second exposure is where symptoms begin. When you encounter the allergen again, it latches onto those waiting antibodies and triggers the mast cells to burst open in a process called degranulation. This floods the surrounding tissue with histamine and other inflammatory chemicals called leukotrienes. Histamine is what makes your nose run, your eyes water, your skin break out in hives, or, in severe cases, your throat swell shut. The whole cascade can fire within minutes.
Not All Allergic Reactions Work the Same Way
The antibody-driven reaction described above is the most common and most recognizable type. But some allergic reactions, particularly to foods, follow a different path entirely. Instead of antibodies triggering mast cells, immune cells called T cells attack the allergen directly in the gut lining, releasing inflammatory signals that increase intestinal permeability and cause fluid shifts. This can lead to severe vomiting and diarrhea hours after eating, rather than the immediate hives or throat swelling people typically associate with food allergies.
These delayed reactions are harder to diagnose because standard allergy blood tests look for the antibodies involved in the faster type of reaction. When those come back negative but a child keeps reacting to a specific food, a T-cell driven condition may be the cause.
Common Allergy Triggers
The substances that trigger allergies fall into a few broad categories, though the list is long:
- Airborne allergens: tree, grass, and weed pollens; dust mites; mold spores; pet dander (proteins shed from skin, saliva, and urine of cats, dogs, and other animals).
- Food allergens: the “Big 9” in the U.S. are milk, eggs, fish, shellfish, tree nuts, peanuts, wheat, soybeans, and sesame. Some processed foods like hot dogs and chicken nuggets contain hidden allergenic ingredients such as dry milk powder or hydrolyzed wheat protein.
- Insect venom: stings from bees, wasps, hornets, and fire ants. Any detectable level of venom-specific antibodies in blood testing is considered a positive result because there’s no safe threshold that rules out a dangerous reaction.
- Medications: penicillin and related antibiotics are among the most common drug allergens, along with certain anesthetics and latex.
One increasingly recognized trigger is alpha-gal syndrome, a potentially life-threatening allergy to a sugar molecule found in mammalian meat (beef, pork, lamb, venison) and mammal-derived products like gelatin and cow’s milk. It develops after a tick bite and can cause reactions hours after eating red meat, making it easy to miss.
Poultry allergies are a separate condition, but people allergic to hen’s eggs may cross-react to duck and quail eggs as well as chicken meat itself.
Why Some People Develop Allergies and Others Don’t
Genetics play an obvious role. If both your parents have allergies, your risk is significantly higher than someone with no family history. But genes alone don’t explain why allergy rates have surged in industrialized countries over the past several decades while remaining lower in rural and less-developed regions.
The leading explanation centers on microbial exposure during early life. The idea, originally called the hygiene hypothesis, has evolved considerably since it was first proposed. Current evidence points not to infections but to everyday contact with a diverse range of friendly microorganisms. These “old friends,” microbes that have been present throughout human evolution, appear to train the immune system to distinguish real threats from harmless proteins like food and pollen.
This training begins before birth. Maternal microbes start colonizing a baby’s gut while it’s still in the womb, and the process continues through the birth canal, breastfeeding, and every subsequent interaction with family members, pets, dirt, and shared toys. Children raised in environments with less microbial diversity, think urban apartments versus farms, fewer siblings, more antibiotic use, tend to develop allergies at higher rates. It’s not that they lack exposure to germs. It’s that their exposure to the microbial world is far more limited than it once was.
Climate Change Is Making Pollen Allergies Worse
If your seasonal allergies feel worse than they did a decade ago, you’re probably right. Rising temperatures, higher atmospheric carbon dioxide levels, and fewer frost days are shifting pollen seasons earlier in the year and stretching them longer. Warmer air and more CO2 also stimulate plants to produce more pollen overall, increasing the amount suspended in the air on any given day. Changes in precipitation patterns add another variable, with some regions seeing spikes in mold growth after heavier rainfall events.
Early Food Introduction Can Reduce Risk
For years, parents were told to delay introducing allergenic foods to infants. That advice has reversed. Current guidelines from the FDA and the Dietary Guidelines for Americans recommend introducing peanut-containing foods as early as 4 to 6 months for infants at high risk (those with severe eczema, egg allergy, or both). This early exposure significantly reduces the chance of developing a peanut allergy later.
For high-risk infants, a blood test or skin prick test is typically recommended first to determine whether peanut can be safely introduced at home or whether it should happen under medical supervision. The same principle of early, gradual introduction is being studied and applied to other common food allergens like egg and cow’s milk.
How Allergies Are Identified
Allergy testing usually involves either a skin prick test or a blood test that measures levels of allergen-specific antibodies. Blood tests are generally considered positive when antibody levels exceed 0.35 kU/L, though sensitization to some airborne allergens can occur at levels as low as 0.12 kU/L.
For food allergies, the picture is more nuanced. Researchers have established antibody thresholds that predict with high confidence whether someone will react to a given food. Peanut-specific antibody levels of 14 kU/L or higher predict a clinical reaction with near certainty. Egg levels above 7 kU/L in children older than 2 carry a 98% predictive value. But for foods like soybean and wheat, even high antibody levels are less reliable predictors, meaning a positive blood test doesn’t always mean a true allergy.
This is why allergists often confirm results with an oral food challenge, where you eat the suspected food in gradually increasing amounts under observation. It remains the most definitive way to confirm or rule out a food allergy, especially when blood tests fall in an ambiguous range.