An allergic reaction is an overreaction of your immune system to an allergen, a substance that is harmless to most people.

Your immune system normally protects you against bacteria, viruses and other foreign invaders. In an allergic reaction, the allergen causes immune cells called mast cells to release histamine and other substances. These substances can cause sneezing, watery eyes, itching and other symptoms.

Usually, an allergic reaction is bothersome, but not life-threatening. Rarely, an allergic reaction can lead to anaphylaxis. This is a life-threatening response involving lowered blood pressure and trouble breathing.

Call 9-1-1 if you have any of the following signs of a severe allergic reaction:

  • Difficulty breathing
  • Chest pain or tightness
  • Sudden onset of wheezing
  • Swollen lips, tongue or throat
  • Bluish tint to skin or lips
  • Fainting, dizziness or confusion
  • Allergic symptoms that start very shortly after an exposure or are severe

Also seek emergency medical care if any of the following are present:

  • Swelling in the face or neck
  • Itching in the throat
  • Severe itching or hives
  • Abdominal pain, cramping, nausea or vomiting that starts soon after an exposure

Make sure that you call your doctor if you have an allergic reaction, even if the symptoms are mild.

The most common allergic conditions include:

1. Allergic asthma.

When asthma symptoms are caused by allergies, it is called allergic asthma. Symptoms of allergic asthma are at least partly reversible with medication. Symptoms may include:

  • Coughing
  • Wheezing
  • Shortness of breath or rapid breathing
  • Chest tightness
  • Some fatigue and slight chest pain 

2. Allergic conjunctivitis.

This condition is a type of "pink eye." The form of pink eye caused by allergens, such as pollen, mold and pet dander, is called allergic conjunctivitis. Symptoms include itchy and watery eyes. Allergic conjunctivitis is not contagious like the other types of pink eye, which are caused by bacteria or viruses. Allergic conjunctivitis is often linked with other allergic conditions, such as eczema, hay fever and asthma.

3. Allergic contact dermatitis.

In allergic contact dermatitis the skin breaks out in a rash after it comes into direct contact with normally harmless substances. This condition can be caused by chemicals found in plants or products. Poison ivy is one example of common plant allergen that can cause it. Hair dye, nail products and detergents are other examples of allergens that can cause an allergic rash.

4. Atopic dermatitis (eczema).

Eczema is a skin condition that is fairly common in infants and children. In eczema, the skin breaks out in an itchy rash that can be made worse by scratching. Eczema is often related to the development of other allergic conditions, such as rhinitis and asthma.

5. Foodallergies.

Food allergies are most common in young children who frequently outgrown them. Symptoms include:

  • Itching
  • Swelling of lips or tongue
  • Throat tightness
  • Hoarseness
  • Nausea and vomiting
  • Diarrhea
  • Itchy eyes
  • Rarely, symptoms of anaphylaxis, especially to peanuts and tree nuts

6. Hay fever.

Hay fever is a type of allergic rhinitis. With this, the nose is runny and stuffed up due to allergens. Allergic rhinitis that occurs in the spring, summer or fall and is due to pollen is called hay fever or seasonal allergies. Allergic rhinitis that occurs year-round can be due to indoor or outdoor allergens including dust, molds and animal dander. The allergic reaction is characterized by:

  • Sneezing
  • Congestion and/or runny nose
  • Itchy nose, eyes, mouth and throat

7. Insect allergies.