An allergic reaction is your immune system overreacting to a substance that is normally harmless, like pollen, peanuts, or pet dander. Instead of ignoring these substances, your body treats them as dangerous invaders and launches an inflammatory attack. About one in three American adults has at least one diagnosed allergic condition, whether that’s seasonal allergies, eczema, or a food allergy.
How Your Immune System Creates an Allergic Reaction
Allergies develop in two stages. The first time you encounter an allergen, nothing dramatic happens on the outside. Internally, though, your immune cells (called B-cells) identify the substance as a threat and produce antibodies called IgE. These antibodies spread throughout your body and attach to immune cells called mast cells, which sit in your skin, lungs, gut lining, and other tissues. At this point, you’re “sensitized,” meaning your body is now primed to react the next time it sees that substance.
The second exposure is when symptoms hit. When the allergen enters your body again, your mast cells recognize it immediately and release a flood of chemicals, most notably histamine. Histamine makes blood vessels leak fluid, which causes swelling, congestion, and watery eyes. It triggers nerve endings, producing itching. It constricts the muscles around your airways, making it harder to breathe. And in the gut, it can cause cramping, vomiting, and diarrhea. All of these responses evolved to fight parasites, but in an allergic person, they fire against things like tree pollen or shrimp.
Common Symptoms by Trigger Type
Allergic reactions look different depending on what triggered them and where the allergen enters your body.
Airborne allergens (pollen, dust mites, mold, pet dander) typically cause sneezing, a stuffy or runny nose, and watery, red, swollen eyes. These are the classic “hay fever” symptoms.
Food allergens can produce a wider range of effects: tingling in the mouth, hives, swelling of the lips or tongue, nasal congestion, stomach cramps, vomiting, or diarrhea. Reactions can start within minutes of eating the food or take up to a couple of hours.
Insect stings often cause pain and large swelling at the sting site, but a true allergic reaction goes further, producing hives across the body, skin flushing, chest tightness, wheezing, or shortness of breath.
Medications can trigger hives, rash, facial swelling, wheezing, dizziness, or digestive symptoms. Antibiotics, pain relievers, and anesthesia agents are among the more common culprits.
The Nine Major Food Allergens
While any food can theoretically cause an allergic reaction, nine account for the vast majority of food allergy cases in the United States. Federal labeling law requires these to be clearly identified on packaged food:
- Milk
- Eggs
- Peanuts
- Tree nuts (almonds, walnuts, pecans, and others)
- Fish (bass, flounder, cod, and others)
- Crustacean shellfish (shrimp, crab, lobster)
- Wheat
- Soybeans
- Sesame (added to the list in 2021)
Anaphylaxis: The Most Serious Reaction
Anaphylaxis is a severe, whole-body allergic reaction that can be life-threatening. It happens when the immune system releases such a massive wave of chemicals that multiple organ systems are affected at once. Blood pressure drops suddenly, the airways narrow, and the tongue or throat can swell enough to block breathing. A person in anaphylaxis may have a weak, rapid pulse, feel dizzy or faint, and lose consciousness.
Other signs include widespread hives, flushed or pale skin, nausea, vomiting, and diarrhea. Anaphylaxis can be triggered by foods, insect stings, medications, or latex. It requires immediate treatment with epinephrine, which is the only drug that can reverse the reaction quickly. Epinephrine auto-injectors are designed to be used by the person having the reaction or a bystander, and they’re injected into the outer thigh (never the buttock, hands, or feet). The standard dose for adults and children over about 66 pounds is 0.3 mg; smaller children receive a lower dose.
Allergy vs. Intolerance
People often confuse allergies with intolerances, but the distinction matters. A true allergy involves the immune system producing IgE antibodies and releasing histamine. It can affect your skin, lungs, heart, and gut simultaneously, and it carries the risk of anaphylaxis. Even a tiny amount of the trigger can set it off.
A food intolerance, by contrast, typically affects only the digestive system. Lactose intolerance, for example, causes bloating, gas, and diarrhea because your body lacks the enzyme to break down milk sugar. It’s uncomfortable, but it’s not dangerous in the way an allergy can be, and the severity usually depends on how much you consumed. Celiac disease is a notable gray area: it involves the immune system but operates through a different mechanism than a classic allergy.
Who Gets Allergies
Allergies are remarkably common. CDC data from 2024 shows that 25.2% of American adults have a diagnosed seasonal allergy, 7.7% have eczema, and 6.7% have a diagnosed food allergy. Women are consistently more affected across all three categories. Nearly 30% of women report seasonal allergies compared to about 21% of men, and women are roughly 60% more likely to have a diagnosed food allergy.
Genetics play a strong role. If one or both of your parents have allergies, you’re more likely to develop them, though not necessarily to the same triggers. Allergies can appear at any age, and it’s possible to develop a new allergy as an adult even if you’ve never had one before.
How Allergies Are Diagnosed
If you suspect an allergy, testing can confirm which specific substances your immune system reacts to. The two main approaches are skin tests and blood tests.
A skin prick test involves placing tiny amounts of suspected allergens on your skin (usually your forearm or back) and lightly pricking the surface. If you’re allergic, a small raised bump appears within about 15 minutes. It’s quick, inexpensive, and gives results on the spot.
Blood tests measure the level of allergen-specific IgE antibodies in your blood. A result above 0.35 kU/L is generally considered positive, though sensitization to some airborne allergens can show up at even lower levels. For food allergies, higher IgE levels correspond to a greater likelihood of a true clinical reaction, but no blood test alone can definitively confirm a food allergy. In some cases, a supervised oral food challenge, where you eat small amounts of the suspected food under medical observation, is the most reliable way to know for sure.
Managing Allergic Reactions
The most effective strategy for any allergy is avoiding the trigger. For food allergies, that means reading labels carefully, asking about ingredients at restaurants, and knowing the nine major allergens by name. For environmental allergies, reducing exposure through air filters, keeping windows closed during high-pollen days, and showering after outdoor activity can make a significant difference.
When avoidance isn’t enough, antihistamines block histamine from binding to receptors in your tissues, which reduces sneezing, itching, and hives. Nasal corticosteroid sprays reduce inflammation in the nasal passages for people with persistent seasonal or year-round allergies. For insect venom and some environmental allergies, immunotherapy (allergy shots or under-the-tongue tablets) gradually retrains your immune system to tolerate the allergen over months to years.
Anyone who has experienced anaphylaxis, or who has a known allergy to a food, insect venom, or medication that carries anaphylaxis risk, should carry an epinephrine auto-injector at all times. Antihistamines alone cannot stop anaphylaxis.