Allergies are hypersensitive immune responses to substances that either enter or come in contact with the body, such as pet dander, pollen or bee venom.
A substance that causes an allergic reaction is called an "allergen". Allergens can be found in food, drinks or the environment.
Most allergens are harmless, i.e. the majority of people are not affected by them.
If you are allergic to a substance, such as pollen, your immune system reacts to it as if it were a pathogen (a foreign harmful substance), and tries to destroy it.
Contents of this article:
Fast facts on allergies
Here are some key points about allergies. More detail and supporting information is in the main article.
- Allergies are the result of an inappropriately large immune response
- Some of the most common allergens are dust, pollen and nuts
- An estimated 1 in 5 Americans have an allergy
- Allergies have a range of symptoms that can include sneezing, peeling skin and vomiting
- Anaphylaxis is a serious allergic reaction that can be life-threatening
- There are a number of risk factors for allergies, including a family history
- If you already have an allergy, you are more likely to develop an allergy to something else
- In theory, any food has the potential to be an allergen
- To diagnose an allergy, a clinician may take a blood sample.
What is an allergy?
An estimated 1 in 5 Americans have an allergy.
The number of people worldwide with allergies is increasing. According to Allergy UK, about 30-40% of people have an allergy at some stage in their lives. Some years ago, this increase was only apparent in industrialized nations. However, middle-income nations are now reporting higher rates of allergies across their populations.
The steepest increase in allergies has been observed in children, particularly food allergies.
A team of researchers from Northwestern University Feinberg School of Medicine reported in Pediatrics that about 8% of American children have some kind of food allergy. 38.7% of those with food allergies have a history of anaphylaxis (severe allergic reactions), and 30.4% are allergic to more than one food.
Researchers from St. Luke's Roosevelt Hospital Center, New York, found that foreign-born children who live in the USA have a lower risk of allergies. This risk grows the longer they remain in America.
Symptoms of allergies
When a person with an allergy comes into contact with an allergen, the allergic reaction is not immediate. The immune system gradually builds up sensitivity to the substance before overreacting to it.
The immune system needs time to recognize and remember the allergen. As it becomes sensitive to it, it starts making antibodies to attack it - this process is called sensitization.
Sensitization can take from a few days to several years. In many cases the sensitization process is not completed and the patient experiences some symptoms but never a full allergy.
When the immune system reacts to an allergen, there is inflammation and irritation. Signs and symptoms depend on the type of allergen. Allergic reactions may occur in the gut (digestive system), skin, sinuses, airways, eyes, and nasal passages.
Allergies from dust and pollen may have the following symptoms:
- Blocked nose
- Itchy eyes
- Itchy nose
- Runny nose
- Swollen eyes
- Watery eyes
Skin reactions, as in eczema (atopic dermatitis) may include:
- Flaking skin
- Itchy skin
- Peeling skin
- Red skin, rashes.
Food allergies may include several types of reactions:
- Tongue swelling
- Tingling in the mouth
- Swelling of the lips
- Swelling of the face
- Swelling in the throat
- Stomach cramps
- Shortness of breath
- Rectal bleeding (in children, rare in adults)
- Itchiness in the mouth
- Anaphylaxis - a very severe, often life-threatening allergic reaction.
The following allergic reactions are possible after an insect sting:
- Swelling where the sting occurred
- Sudden drop in blood pressure
- Skin itching
- Shortness of breath
- Hives - a red and very itchy rash that spreads
- Chest tightness
The following may be signs of an allergic reaction to medication:
- Swollen tongue
- Swollen lips
- Swelling of the face
- Skin rash
Symptoms of anaphylaxis
Anaphylaxis is a serious allergic reaction of rapid onset. Anaphylaxis can be life-threatening and must be treated as a medical emergency.
This type of allergic reaction presents several different symptoms which can appear minutes or hours after exposure to the allergen. If the exposure is intravenous, onset is usually between 5 to 30 minutes. A food allergen will take longer.
Anaphylaxis is a medical emergency.
Researchers from the University of Manitoba, Canada, reported in The Journal of Allergy & Clinical Immunology that the most commonly affected areas in anaphylaxis are the skin (80-90%), respiratory (70%), gastrointestinal (30-45%), cardiovascular 10-45%) and the central nervous system (10-15%). In most cases two areas are affected simultaneously.
Anaphylaxis - skin symptoms
Hives all over the body, flushing and itchiness. The affected tissues may also become swollen (angioedema). Some patients may experience a burning sensation on the skin.
In about 20% of cases, there is swelling of the tongue and throat.
If the skin has a strange bluish color, it could be a sign of hypoxia (lack of oxygen).
Some patients may experience a runny nose. The membrane that covers the front of the eye and the inside of the eyelid (conjunctiva) may become inflamed.
Anaphylaxis - respiratory symptoms
- Shortness of breath
- Wheezing - caused by bronchial muscle spasms
- Stridor - a high-pitched vibrating wheezing sound when breathing. Caused by upper airway obstruction due to swelling
- Odynophagia - pain when swallowing
Anaphylaxis - cardiovascular symptoms
Coronary artery spasm - sudden tightening of the muscle in the artery wall (temporary) due to cells in the heart that release histamine. This can lead to myocardial infarction (heart attack), dysrhythmia (abnormal heart rhythm), or cardiac arrest (heart stops).
Low blood pressure can cause the heart rate to accelerate. In some cases a slow heart rate can occur as a result of low blood pressure (Bezold-Jarisch reflex).
Patients whose blood pressure suddenly drops can feel lightheaded and dizzy. Some may lose consciousness. In some rare cases, the only sign of anaphylaxis might be low blood pressure.
Anaphylaxis - gastrointestinal symptoms
- Abdominal cramps
- Loss of bladder control
- Pelvic pain (like uterine cramps).
Patients may also have a sense of impending doom.
Causes of allergies
Allergies are caused by an over-sensitive immune system.
The immune system of a person with an allergy reacts to the allergen as though it were a harmful pathogen - such as an undesirable bacterium, virus, fungus or toxin.
However, the allergen is not harmful. The immune system has simply become oversensitive to that substance.
When the immune system reacts to an allergen, it releases immunoglobulin E (IgE), a type of antibody. IgE is released to destroy the allergen.
IgE causes chemicals in the body to be produced. These chemicals cause the allergic reaction.
One of these chemicals is called histamine. Histamine causes tightening of the muscles, including those in the airways and the walls of blood vessels. It also makes the lining of the nose produce more mucus.
People with allergies blame the allergen for their symptoms - a friend's pet, pollen or dust mites. However, the allergens are not harmful. The problem is not the allergen but the allergic person's immune system which mistakes harmless substances for harmful ones.
Risk factors for allergies
In medicine, a risk factor is something that raises the risk of developing a disease or condition. This risk can come from something a person does. For example, smoking is a risk factor for lung disease. It can also be something you are born with. For example, if your mother had breast cancer, her daughter has a higher risk of developing breast cancer too. A family history of breast cancer is a risk factor.
Below are some risk factors associated with allergies:
- A family history of asthma - if your parents, grandparents or siblings have/had asthma, your risk of having an allergy is higher
- A family history of allergies - if a close relative has/had an allergy, your risk of having an allergy yourself is greater
- Being a child - a child is much more likely to have an allergy than an adult. On a positive note, this means that many children outgrow their allergies
- Having asthma - people with asthma are significantly more likely to develop allergies
- Not enough sunlight exposure - scientists from the European Centre for Environment & Human Health, together with researchers from various Australian centers found that children living in areas with less sunlight had higher rates of allergies
- Having an allergy - if you already have an allergy, there is a greater risk that you will develop an allergy to something else
- C-section babies - a team from the Henry Ford Hospital reported that C-section babies have a considerably higher risk of developing allergies compared to those born naturally
- Chemicals used in water purification - Dr. Elina Jerschow, a fellow of the American College of Allergy, Asthma and Immunology, said that pesticides in tap water could be partly to blame for the increased food allergy rates in the USA.
On the next page, we look at the most common allergies, their diagnosis and treatment.