An allergy is your immune system reacting to a substance that is normally harmless, like pollen, pet dander, or peanuts, as though it were a dangerous invader. Roughly 20 to 30 percent of the global population has at least one allergy, and that number is climbing, particularly in developed countries and urban areas. The reaction can be as mild as a runny nose or as severe as a life-threatening emergency called anaphylaxis.
How an Allergic Reaction Works
An allergy unfolds in two stages. The first is sensitization: the first time your body encounters a particular substance (the allergen), your immune system mistakenly flags it as a threat and produces a specific type of antibody called IgE. These IgE antibodies latch onto mast cells, which are immune cells packed with inflammatory chemicals and stationed throughout your skin, airways, and gut. At this point you feel nothing. Your body is simply primed.
The second stage is the reaction itself. The next time that allergen enters your body, it binds to the IgE antibodies already sitting on your mast cells. This causes the mast cells to burst open within seconds, flooding the surrounding tissue with histamine and other inflammatory chemicals. Histamine increases blood flow to the area, makes blood vessels leakier, and causes smooth muscle to contract. That’s what produces the swelling, redness, itching, and mucus you feel during an allergic reaction.
There’s also a delayed wave. Eight to twelve hours after the initial reaction, mast cells release a second round of chemicals that recruit additional immune cells to the area. This late-phase response is why allergy symptoms can linger or worsen hours after you’ve been exposed, even if you’re no longer near the trigger.
Common Allergy Triggers
Allergens generally fall into a few broad categories:
- Airborne allergens: pollen (from trees, grasses, and weeds), dust mites, mold spores, and animal dander. These enter through the nose or eyes and typically cause respiratory and nasal symptoms.
- Food allergens: cow’s milk, eggs, peanuts, tree nuts, wheat, soy, fish, shellfish, and seeds account for the vast majority of food-related allergic reactions. Heat-stable proteins in some of these foods survive cooking and digestion, making them more likely to trigger systemic reactions.
- Insect stings: venom from bees, wasps, hornets, and fire ants can trigger reactions ranging from local swelling to anaphylaxis.
- Contact allergens: certain metals (nickel), latex, and chemicals in cosmetics or cleaning products cause delayed skin reactions rather than the immediate IgE-driven type.
- Medications: antibiotics like penicillin and nonsteroidal anti-inflammatory drugs are among the most common drug triggers.
Food allergy prevalence is notable: in Europe, nearly 20 percent of the population reports some form of food allergy. Pollen allergies are also increasing globally, with more people developing respiratory symptoms tied to seasonal pollen exposure.
What Allergy Symptoms Feel Like
Because histamine targets multiple organ systems, allergy symptoms depend on where the allergen enters your body and how your immune system responds. Upper respiratory symptoms include sneezing, a stuffy or runny nose, and swelling around the eyes or lips. Lower respiratory symptoms include coughing, wheezing, and chest tightness. On the skin, allergies cause hives, flushing, or itchy rashes. In the digestive system, they can trigger vomiting, cramping, and diarrhea.
Most allergic reactions stay mild and localized. Seasonal pollen allergies, for example, rarely go beyond nasal and eye symptoms. Food allergies, insect stings, and drug allergies carry more risk of widespread reactions.
Anaphylaxis
Anaphylaxis is the most dangerous form of allergic reaction and requires immediate treatment. It typically involves two or more body systems at once: skin symptoms like hives or flushing paired with breathing difficulty, a sudden drop in blood pressure, or severe gastrointestinal distress. Symptoms develop within minutes to hours of exposure. A hallmark sign is the combination of a skin reaction with either trouble breathing or feeling faint or collapsing. Anaphylaxis can occur even without skin symptoms if there is a sudden onset of low blood pressure or airway constriction after contact with a known allergen.
Epinephrine, delivered through an auto-injector, is the first-line treatment. It works by reversing the blood vessel dilation and airway constriction that make anaphylaxis dangerous. Anyone who has been prescribed an auto-injector should carry it at all times, and people nearby should know how to use it. An expired auto-injector should not be used and needs to be replaced.
Why Some People Develop Allergies
Genetics play a clear role. If one or both of your parents have allergies, your risk is significantly higher, and studies of identical twins show strong concordance rates. But genes alone don’t explain the pattern. The global rise in allergies over the past several decades has been far too rapid to be genetic; something in the environment has changed.
The leading explanation is the hygiene hypothesis. It proposes that children growing up in highly sanitized environments with less exposure to bacteria, parasites, and other microbes don’t develop the same immune regulation as children in less sanitized settings. Without that early microbial training, the immune system is more likely to overreact to harmless substances. Supporting this: children raised on farms or exposed to cowsheds and higher levels of natural bacterial compounds in early life have lower rates of allergies and asthma. The widespread use of antibiotics, smaller family sizes, and less time spent outdoors all fit this pattern.
Environmental pollution also appears to be a factor. The increasing prevalence of allergies in urban and industrialized areas tracks closely with air pollution levels, which can both damage airway tissue and make the immune system more reactive.
How Allergies Are Diagnosed
The two most common tests for immediate allergies are skin prick testing and blood testing. In a skin prick test, tiny drops of liquid allergen are placed on your forearm, and a small lancet pricks the skin beneath each drop. After 15 to 20 minutes, a raised bump (wheal) of 3 millimeters or more indicates a positive reaction. The test includes a histamine drop as a positive control and a saline drop as a negative control to make sure the results are reliable.
If skin testing isn’t practical, perhaps because of a widespread skin condition or because you can’t stop taking antihistamines, a blood test can measure the level of IgE antibodies against specific allergens in your bloodstream. Both tests identify what your immune system is sensitized to, but neither one alone confirms a clinical allergy. Your doctor will interpret the results alongside your symptom history.
For delayed-type reactions, like contact allergies to metals or chemicals, a patch test is used instead. Small adhesive patches containing potential allergens are applied to your back and left in place for 48 hours before the skin is evaluated for a reaction.
Treatment and Long-Term Management
For most people, allergy management starts with avoidance: identifying your triggers and reducing exposure. That might mean using dust mite covers on bedding, keeping windows closed during high pollen counts, or reading food labels carefully.
When avoidance isn’t enough, antihistamines are the most widely used treatment. They work by blocking histamine from binding to receptors in your tissues, which reduces sneezing, itching, and runny nose. Nasal corticosteroid sprays reduce inflammation in the nasal passages and are particularly effective for persistent nasal allergies. For allergic asthma, inhaled medications that open the airways or reduce inflammation are the standard approach.
Allergen immunotherapy is the only treatment that changes the underlying immune response rather than just managing symptoms. It involves gradually exposing your body to increasing doses of the allergen over months to years, retraining the immune system to tolerate it. This can be done through regular injections (allergy shots) or daily tablets or drops placed under the tongue. A typical course runs about three years. In children and adolescents with pollen-triggered allergies, a three-year course has been shown to prevent the development of asthma for up to two years after treatment ends. Immunotherapy also reduces the need for daily allergy medications in many patients and can provide lasting improvement in symptoms even after treatment stops.