Hives are caused by the release of histamine and other inflammatory chemicals from specialized immune cells in your skin called mast cells. When something triggers these cells, they dump histamine into surrounding tissue, which makes tiny blood vessels leak fluid. That fluid pools just under the skin’s surface, creating the raised, itchy welts you see. The triggers range from allergic reactions and infections to physical contact and stress, and in many cases, no clear cause is ever identified.
How Hives Form Under the Skin
Mast cells sit throughout your skin, essentially acting as sentinels for the immune system. Their surface is covered in different types of receptors, which means many different signals can set them off. The classic trigger is an allergic reaction: your immune system produces antibodies that latch onto mast cells, and when the allergen shows up again, those antibodies signal the mast cells to release their contents. But allergies are only one pathway. Mast cells can also be activated by the complement system (part of your body’s infection-fighting machinery), by blood clotting proteins, and by a receptor called MRGPRX2 that responds to certain drugs and chemicals without any allergy involved.
Once histamine hits the surrounding tissue, blood vessels widen and become leaky. Fluid seeps into the upper layers of skin, forming a wheal, the raised bump characteristic of hives. The same process triggers nerve endings, producing that intense itch. Individual hives typically fade within 24 hours as the fluid reabsorbs, but new ones can keep appearing.
Allergic Triggers: Foods and Medications
Food allergies are one of the most recognizable causes. Nine foods account for roughly 90% of all food allergies: peanuts, tree nuts (almonds, walnuts, cashews, pecans, pistachios, hazelnuts, Brazil nuts), milk, eggs, fish, shellfish, soy, wheat, and sesame. Hives from a food allergy usually appear within minutes to two hours of eating the trigger food.
Medications are another major category. Antibiotics (especially penicillin-type drugs), nonsteroidal anti-inflammatory drugs like ibuprofen and aspirin, and blood pressure medications called ACE inhibitors are well-known culprits. Some of these work through a true allergic mechanism, while others activate mast cells directly through the MRGPRX2 receptor, meaning your body doesn’t need prior exposure to react.
Infections Are the Top Cause in Children
Viral infections are actually the most common cause of widespread hives, especially in kids. The hives show up alongside other signs of illness like fever, cough, or diarrhea and typically last about three days. This is not an allergic reaction. The immune system’s response to the virus itself triggers mast cell activation. Common culprits include upper respiratory viruses, stomach bugs, and hepatitis viruses in adults.
Bacterial infections can do the same thing. Strep throat and urinary tract infections are two of the more common bacterial triggers. In these cases, treating the underlying infection usually resolves the hives.
Physical Triggers You Might Not Expect
Some people break out in hives from purely physical stimuli, with no allergen involved at all.
- Dermatographism is the most common type: firm pressure or friction on the skin, even from clothing rubbing during normal activity, causes welts to appear along the line of contact. You can literally “write” on the skin with a fingernail and watch the word swell up. It affects roughly 2 to 5% of the population.
- Cholinergic urticaria produces tiny, pinpoint welts (about 1 mm) surrounded by red flares. It’s triggered by anything that raises your core body temperature: exercise, hot showers, sweating, even anxiety. The nerve chemical acetylcholine, released when you heat up, activates mast cells in the skin.
- Cold urticaria works in the opposite direction. Exposure to cold air, cold water, or cold objects causes rapid itching, redness, and swelling in the exposed area. Swimming in cold water can be particularly dangerous because widespread histamine release may cause a drop in blood pressure.
Other physical triggers include sustained pressure (like from a waistband or carrying heavy bags), vibration, sunlight, and water contact, though these are rarer.
Stress and Hormonal Factors
Emotional stress doesn’t cause hives in the way a peanut allergy does, but it can lower the threshold for mast cell activation. Stress hormones like cortisol and adrenaline influence immune function, and many people with chronic hives notice flares during high-stress periods. Hormonal shifts, particularly around menstrual cycles, pregnancy, or thyroid dysfunction, can also play a role. About 14% of people with chronic hives have antibodies against their own thyroid, compared to 3 to 6% of the general population.
Acute vs. Chronic Hives
The six-week mark is the dividing line. Hives that come and go over less than six weeks are classified as acute. These usually have a clear trigger: a food, a medication, an infection, an insect sting. Most cases of acute hives resolve on their own.
Hives lasting longer than six weeks are chronic. This is where things get frustrating. In fewer than 30% of chronic cases is a specific trigger ever identified. A large portion of chronic hives are now understood to be autoimmune in nature: your immune system produces antibodies that mistakenly activate your own mast cells. The blood clotting system also appears to be involved in about half of severe chronic cases, with markers of clot activation tracking closely with disease severity.
How Hives Are Treated
The first-line treatment is a second-generation antihistamine, the non-drowsy type like cetirizine, loratadine, or fexofenadine. These block histamine from reaching its receptors in the skin. For many people with mild or occasional hives, a standard dose is enough.
If a standard dose doesn’t control symptoms, guidelines recommend increasing the dose up to four times the usual amount. This higher dosing is considered safe and is the standard next step before trying other therapies. For chronic hives that don’t respond even to high-dose antihistamines, a biologic injection that targets the antibody pathway (omalizumab) is the main escalation option. It works by intercepting the immune signals before they reach mast cells.
Identifying and avoiding your specific trigger, when one exists, is obviously the most effective long-term strategy. Keeping a symptom diary that tracks foods, activities, stress levels, and medications can help you and your doctor spot patterns.
When Hives Signal an Emergency
Hives alone, while uncomfortable, are not dangerous. But hives appearing alongside other symptoms can signal anaphylaxis, a severe allergic reaction that can be fatal without treatment. Warning signs include throat or tongue swelling, difficulty breathing or wheezing, a rapid or weak pulse, dizziness or fainting, nausea or vomiting, and flushed or pale skin. If hives appear with any of these symptoms, use an epinephrine auto-injector if available and get to an emergency room immediately. Even if symptoms improve after epinephrine, a second wave of symptoms (called biphasic anaphylaxis) can occur, so emergency monitoring is still necessary.