Allergies occur as a reaction of your immune system to some substances – such as pollen, bee venom, dust or a food – which normally do not cause any reactions in most people.

Immune system produces substances named as antibodies. Immune systems of people who have allergies produces antibodies that identify allergens as harmful, even though thay are not. As a result of the contact with a particular allergen, people who have allergies may have inflammations on skin, sinuses, airways or digestive systems.

The degree of severity of allergies differ from person to person. They can cause minor irritation for some, while for some others they can be a cause of anaphylaxis – a potentially life*threatening emergency.

Symptoms

Allergy symptoms, which depend on the substance involved, can affect your airways, sinuses and nasal passages, skin, and digestive system. Allergic reactions can range from mild to severe. In some severe cases, allergies can trigger a life-threatening reaction known as anaphylaxis.

Hay fever, also called allergic rhinitis, can cause:

Sneezing

Itching of the nose, eyes or roof of the mouth

Runny, stuffy nose

Watery, red or swollen eyes (conjunctivitis)

A food allergy can cause:

Tingling in the mouth

Swelling of the lips, tongue, face or throat

Hives

Anaphylaxis

An insect sting allergy can cause:

A large area of swelling (edema) at the sting site

Itching or hives all over the body

Cough, chest tightness, wheezing or shortness of breath

Anaphylaxis

A drug allergy can cause:

Hives

Itchy skin

Rash

Facial swelling

Wheezing

Anaphylaxis

Atopic dermatitis, an allergic skin condition also called eczema, can cause skin to:

Itch

Redden

Flake or peel

Anaphylaxis

Some types of allergies, including allergies to foods and insect stings, can trigger a severe reaction known as anaphylaxis. A life-threatening medical emergency, anaphylaxis can cause you to go into shock. Signs and symptoms of anaphylaxis include:

Loss of consciousness

A drop in blood pressure

Severe shortness of breath

Skin rash

Lightheadedness

A rapid, weak pulse

Nausea and vomiting

When to see a doctor

You might see a doctor if you have symptoms you think are caused by an allergy, and over-the-counter allergy medications don't provide enough relief. If you have symptoms after starting a new medication, call the doctor who prescribed it right away.

For a severe allergic reaction (anaphylaxis), your local emergency number or seek emergency medical help. If you carry an epinephrine auto-injector (Auvi-Q, EpiPen, others), give yourself a shot right away.

Even if your symptoms improve after an epinephrine injection, you should go to the emergency department to make sure symptoms don't return when the effects of the injection wear off.

If you've had a severe allergy attack or any signs and symptoms of anaphylaxis in the past, make an appointment to see your doctor. Evaluation, diagnosis and long-term management of anaphylaxis are complicated, so you'll probably need to see a doctor who specializes in allergies and immunology.

Causes

An allergy starts when your immune system mistakes a normally harmless substance for a dangerous invader. The immune system then produces antibodies that remain on the alert for that particular allergen. When you're exposed to the allergen again, these antibodies can release a number of immune system chemicals, such as histamine, that cause allergy symptoms.

Common allergy triggers include:

Airborne allergens, such as pollen, animal dander, dust mites and mold

Certain foods, particularly peanuts, tree nuts, wheat, soy, fish, shellfish, eggs and milk

Insect stings, such as from a bee or wasp

Medications, particularly penicillin or penicillin-based antibiotics

Latex or other substances you touch, which can cause allergic skin reactions

Risk factors

You might be more likely to develop an allergy if you:

Have a family history of asthma or allergies, such as hay fever, hives or eczema

Are a child

Have asthma or another allergic condition

Complications

Having an allergy increases your risk of certain other medical problems, including:

Anaphylaxis. If you have severe allergies, you're at increased risk of this serious allergy-induced reaction. Foods, medications and insect stings are the most common triggers of anaphylaxis.

Asthma. If you have an allergy, you're more likely to have asthma — an immune system reaction that affects the airways and breathing. In many cases, asthma is triggered by exposure to an allergen in the environment (allergy-induced asthma).

Sinusitis and infections of the ears or lungs. Your risk of getting these conditions is higher if you have hay fever or asthma.

Prevention

Preventing allergic reactions depends on the type of allergy you have. General measures include the following:

Avoid known triggers. Even if you're treating your allergy symptoms, try to avoid triggers. If, for instance, you're allergic to pollen, stay inside with windows and doors closed when pollen is high. If you're allergic to dust mites, dust and vacuum and wash bedding often.

Keep a diary. When trying to identify what causes or worsens your allergic symptoms, track your activities and what you eat, when symptoms occur and what seems to help. This may help you and your doctor identify triggers.

Wear a medical alert bracelet. If you've had a severe allergic reaction, a medical alert bracelet (or necklace) lets others know that you have a serious allergy in case you have a reaction and you're unable to communicate.

Diagnosis

To evaluate whether you have an allergy, your doctor will likely:

Ask detailed questions about signs and symptoms

Perform a physical exam

Have you keep a detailed diary of symptoms and possible triggers

If you have a food allergy, your doctor will likely:

Ask you to keep a detailed diary of the foods you eat

Ask if you've stopped eating the suspected food during the allergy evaluation

Your doctor might also recommend one or both of the following tests. However, be aware that these allergy tests can be falsely positive or falsely negative.

Skin test. A doctor or nurse will prick your skin and expose you to small amounts of the proteins found in potential allergens. If you're allergic, you'll likely develop a raised bump (hive) at the test location on your skin.

Blood test. Specific IgE (sIgE) blood testing, commonly called radioallergosorbent test (RAST) or ImmunoCAP testing, measures the amount of allergy-causing antibodies in your bloodstream, known as immunoglobulin E (IgE) antibodies. A blood sample is sent to a medical laboratory, where it can be tested for evidence of sensitivity to possible allergens.

If your doctor suspects your problems are caused by something other than an allergy, other tests might help identify — or rule out — other medical problems.

Treatment

Allergy treatments include:

Allergen avoidance. Your doctor will help you take steps to identify and avoid your allergy triggers. This is generally the most important step in preventing allergic reactions and reducing symptoms.

Medications. Depending on your allergy, medications can help reduce your immune system reaction and ease symptoms. Your doctor might suggest over-the-counter or prescription medication in the form of pills or liquid, nasal sprays, or eyedrops.

Immunotherapy. For severe allergies or allergies not completely relieved by other treatment, your doctor might recommend allergen immunotherapy. This treatment involves a series of injections of purified allergen extracts, usually given over a period of a few years.

Another form of immunotherapy is a tablet that's placed under the tongue (sublingual) until it dissolves. Sublingual drugs are used to treat some pollen allergies.

Emergency epinephrine. If you have a severe allergy, you might need to carry an emergency epinephrine shot at all times. Given for severe allergic reactions, an epinephrine shot (Auvi-Q, EpiPen, others) can reduce symptoms until you get emergency treatment.