An allergy is an immune system reaction to a substance that is usually harmless to most people. The substance that triggers the reaction is called an allergen. Use the links below to read about common allergic conditions and triggers.
Common Allergies and Allergic Conditions
Pollen and indoor triggers
- Pollen Allergy — seasonal symptoms triggered by pollen from trees, grasses or weeds.
- Grass Pollen Allergy — a common cause of seasonal allergic rhinitis.
- House Dust Mite Allergy — allergy to proteins from microscopic mites found in household dust.
- Mold Allergy — allergic symptoms triggered by mold spores.
Animal allergens
- Pet Allergy — reactions to allergens from animals.
- Cat Allergy — allergy to proteins found in cat dander, saliva and other secretions.
- Dog Allergy — allergic reactions to proteins associated with dogs.
Food allergies
- Food Allergy — an immune reaction to a food protein that can range from mild to life threatening.
- Nut Allergy — allergy to peanuts or tree nuts that can cause severe reactions in some people.
- Cow’s Milk Protein Allergy — an immune reaction to proteins in cow’s milk, most often affecting infants and young children.
- Wheat Allergy — an allergic reaction to proteins in wheat and distinct from celiac disease.
- Pollen Food Syndrome — reactions to certain raw fruits, vegetables or nuts in people sensitized to pollen.
Skin reactions and severe allergy
- Nickel Allergy — allergic contact dermatitis triggered by nickel.
- Hives (Urticaria) — raised, itchy welts that can have allergic or nonallergic causes.
- Anaphylaxis — a severe, potentially life-threatening allergic reaction requiring emergency treatment.
Possible Symptoms
Allergic reactions may begin within minutes or develop over several hours. Symptoms can include sneezing, a runny or blocked nose, itchy or watery eyes, hives, swelling, coughing, wheezing, shortness of breath, nausea, vomiting, abdominal symptoms, dizziness or fainting.
Anaphylaxis: A Medical Emergency
Anaphylaxis can cause difficulty breathing, swelling of the tongue or throat, trouble swallowing, sudden dizziness, confusion, collapse or loss of consciousness. Contact your local emergency services immediately if anaphylaxis is suspected. If an epinephrine auto-injector has been prescribed, use it according to the person’s emergency plan while waiting for help.
Diagnosis and Treatment
A healthcare professional may assess symptoms, timing and possible triggers and may use skin-prick testing, blood tests or supervised challenge testing when appropriate. Treatment depends on the allergy and may include avoiding confirmed triggers, antihistamines, nasal corticosteroids, asthma treatment, allergen immunotherapy in selected cases and emergency epinephrine for people at risk of anaphylaxis.
Sources and Further Reading
Last reviewed: 4 September 2026. This page provides general health information and does not replace individual medical assessment or treatment.