The Body's Most Famous False Alarm
About 10% of people say they are allergic to penicillin. Here is the twist: 90% of them turn out not to be. Serious penicillin allergies show up in only about 0.03% of people. Allergies are full of surprises like this, because the whole thing is a case of mistaken identity.
An allergy is an exaggerated immune response. The body treats a normally harmless substance, called an allergen, as if it were a threat. Common allergens include pollen, certain foods, metals, insect stings, medications and latex. The results range from red eyes, sneezing and a runny nose to itchy rashes, coughing and swelling.
Allergies are very common. In the developed world, about 20% of people have hay fever, and food allergy affects 10% of adults and 8% of children. The word itself is just over a century old. Viennese pediatrician Clemens von Pirquet introduced it in 1906, building it from Ancient Greek words meaning "other" and "work".
How the Alarm Gets Wired
An allergy usually takes two meetings. The first time the body meets an allergen, certain immune cells push other cells to make lots of an antibody called immunoglobulin E, or IgE. This IgE travels in the blood and sticks to the surface of mast cells and basophils. Those cells are now sensitized, primed and waiting.
On a later meeting, the allergen latches onto the IgE on those primed cells. That makes the cells empty their tiny storage packets and release histamine and other chemicals. Blood vessels widen, mucus flows, nerves fire and muscles tighten. The result is the runny nose, itching and wheezing people know so well.
The reaction may not end there. A second wave, called the late-phase response, often comes 2-24 hours later, as other white blood cells arrive at the spot. In the most severe case, the reaction spreads through the whole body. This is anaphylaxis, which can affect the skin, breathing, the gut and blood pressure at once, and it is treated as an emergency.
The Usual Suspects, and Some Strange Ones
Food allergies have a short list of main culprits. Just eight foods cause 90% of allergic reactions to food: cow's milk, soy, eggs, wheat, peanuts, tree nuts, fish and shellfish. Egg allergies affect one to two percent of children, and about two-thirds outgrow them by age 5. The trouble is usually in the white, not the yolk.
Dust mite allergy is really a reaction to mite droppings. The mites' guts hold strong digestive enzymes that stay in their waste and set off reactions. Latex has a strange link to food too. People allergic to latex may also react to banana, avocado, kiwifruit and chestnut, because some latex proteins are built a lot like proteins in those plants. This overlap is called cross-reactivity.
The oil in poison ivy, urushiol, is not even a protein. It binds to proteins on skin cells and changes their shape, so the immune system no longer sees those cells as normal. About 80-90% of adults get a rash from just 0.0050 mg of pure urushiol.
Why Allergies Keep Rising
Allergies run strongly in families. Identical twins share the same allergic diseases about 70% of the time, compared with about 40% for non-identical twins. But what gets passed down is the tendency, not the target. Parents who are allergic to peanuts may have children who are allergic to ragweed instead.
Genes alone cannot explain the recent rise in allergies. One popular idea is the hygiene hypothesis. It began with an observation: hay fever and eczema were less common in children from larger families, who were presumably exposed to more germs through their siblings. The idea is that an immune system without enough real germs to fight starts attacking harmless things like pollen. Clinical trials have even tested certain worms as an allergy treatment.
City trees may play a part too. Historically, the trees planted in cities were mostly male, to avoid litter from seeds and fruit. Those male trees push pollen counts up, a pattern horticulturist Tom Ogren has called "botanical sexism".
Testing, Treating and Retraining the Body
To find the culprit, doctors often use a skin prick test. Tiny amounts of suspected allergens are placed into small pricks on the inside of the forearm or on the back. If you are allergic, a red, raised bump usually appears within 30 minutes, a lot like a mosquito bite. Blood tests measure allergen-specific IgE instead, without exposing the person to any allergen at all.
Treatment starts with avoiding the trigger. Medications such as antihistamines and steroids ease symptoms. For severe reactions, adrenaline, also called epinephrine, is recommended, and where medical care is not at hand, an autoinjector device can be used. Another option is immunotherapy, which exposes people to larger and larger amounts of an allergen to change how the immune system responds. It helps with hay fever, asthma and insect stings.
Prevention advice now points to early exposure. After US and UK guidelines in 2015 and 2017 encouraged peanut products for high-risk babies at four to six months old, one 2025 analysis found a 43% drop in peanut allergies, with an estimated 40,000 cases prevented.
