Causes and Risk Factors of Allergies

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Allergies are, essentially, caused by a misguided immune system. In your body's efforts to protect your from harm, it misreads an allergen—a harmless substance, such as pollen or pet dander—as something worth fighting.

Allergic reaction to bee sting on a woman's wrist

HAYKIRDI / Getty Images

The body's response can create mild symptoms such as a cough, sneeze, nasal congestion, or a mild rash; or you might have a severe life-threatening extensive rash or anaphylaxis. There are different mechanisms at play for different kinds of allergies. (Historically there have been four defined types but more recently an expansion of this list has been proposed). Some reactions are mediated by Immunoglobulin E (IgE), some involve other immunoglobulins (IgM or Ig G) and complement, some form antigen-antibody complexes, and some are delayed reactions mediated by T cells.

In addition to the above, allergens can be a variety of different substances including mold, food, medications, insect stings, and metals.

Common Causes

Type 1 immediate hypersensitivity reactions are mediated by immunoglobulin E (IgE). Typical examples include reactions to bee or other insect stings, drugs (such as penicillin), and foods (such as nuts).

During an allergic process, the substance responsible for causing the allergy (allergen) binds to antibodies present on white blood cells in your body, including mast cells and basophils. The cells then release chemicals such as histamine and leukotrienes, resulting in allergic symptoms.

Reactions include skin rash, hives, sneezing, runny nose, nausea, diarrhea, or more serious symptoms such as tongue, lip or throat swelling or having an asthma attack. The most serious form of allergic reactions is anaphylaxis, which involves a whole-body allergic reaction, which can be life-threatening.

The types of symptoms that occur depend on where in the body this reaction takes place. For example, if pollen is inhaled, then nasal allergies may occur. With a food allergen, swallowing the food may result in a whole-body reaction, such as hives or anaphylaxis.

Airborne Allergens

As many as one-third of adults and 40% of children have allergic rhinitis due to airborne allergens. The most common ones are:

  • Weeds
  • Mold
  • Dust mites
  • Grass
  • Pollen
  • Pet dander

Foods

Millions of children and adults in the United States have food allergies. Most allergic reactions occur within seconds to minutes; rarely, the reaction can be delayed a few hours. Reactions can produce skin, nasal, gastrointestinal, and cardiovascular symptoms, as well as anaphylaxis. A small percentage of people with an anaphylactic reaction will have a second reaction hours after the first.

Almost 90% of all food allergies are related to these eight foods:

  • Milk (primarily in infants and small children)
  • Eggs
  • Peanuts
  • Tree nuts
  • Soy
  • Wheat
  • Fish
  • Shellfish

Medications

Reactions to medication are many and varied and can include involvement of any part of the body. Hives or other rashes, itching, cough, wheeze, GI reactions, and anaphylaxis are some examples. The list of medications that cause reactions is very long as well and can include any medication for a given individual. However some classes are more likely to cause reactions than others. Antibiotics, anti-seizure meds, non-steroidal anti-inflammatory drugs (NSAIDS), chemotherapy, and biological medications are some of those classes. Ace-inhibitor drugs (and less commonly the associated ARB med class) can cause angioedema in some people through a different mechanism than the other drugs.

Insect Stings and Bites

Sometimes people can experience more severe allergic reactions to insect stings and bites. The most common insect-related allergic reactions stem from:

bed bugs
Bed bug bites. DermNet / CC BY-NC-ND

Contact Allergens

Contact reactions are a delayed Type IV reaction mediated by T lymphocytes.

There are many irritating chemicals that can cause a skin reaction, but some set off a true allergic reaction when you come in contact with them. The most common ones are:

  • Poison ivy, oak, and sumac
poison ivy
Poison ivy rash.  DermNet / CC BY-NC-ND 
  • Nickel
  • Makeup and personal care products
  • Latex
  • Fragrances
  • Antibacterial ointments
  • Formaldehyde
  • Hair dye
  • Leather tanning chemicals

Another example of a Type IV reaction is Stevens-Johnson syndrome, which involves the skin and mucous membranes. It can be caused by an infection or exposure to a medication.

Genetics

Allergies tend to run in families. You are more at risk if your family history includes people with allergies. This is called being atopic. Your body is more likely than most to see a new allergen as a threat and produce IgE antibodies.

Research is actively underway to identify which genes are responsible for making people more susceptible to allergic diseases. But your genes alone might not determine whether you get allergies, as your environment and when you are exposed to allergens may play a big role.

If you are having allergy symptoms, it can be helpful to give a good family history to your healthcare provider, if possible. Include details of family members who had asthma, hay fever, seasonal allergies, hives, eczema, or severe reactions to insect bites or bee stings.

Lifestyle Risk Factors

There are theories that early exposure to allergens in infancy (such as having a dog in the house) and respiratory infections can help prevent developing allergies. On the other hand, the thinking is that it is good to reduce exposure to dust mites by using allergen-impermeable covers on children's bedding and taking other measures to keep their bedrooms free of dust.

Mothers who smoke during pregnancy are more likely to increase the child's risk of allergies. Secondhand smoke also raises the allergy risk for children and infants.

Breastfeeding is recommended for many reasons. It used to be thought that it helped reduce the risk of the child developing allergies later in life but a 2009-2013 study did not substantiate this.

If you have allergies, avoiding the allergens that trigger them is the key step to preventing allergic reactions. This may mean avoiding outdoor exposure during high pollen seasons, checking carefully for ingredients that can trigger food allergies, and not wearing jewelry that can set off nickel allergies.

A Word From Verywell

The list of what can trigger an allergy is very long. If you are prone to allergies or have a family history of susceptibility, discuss any allergy symptoms you have with your healthcare provider. If you have children or plan to have children, talk to your healthcare provider about the current thinking on allergen exposure and what to look for if you think your child has allergies. Early treatment, especially for children, may reduce the impact allergies will have on their lives.

Frequently Asked Questions

  • What causes an allergy?

    An allergy is an inappropriate immune response to a substance or stimulus that is otherwise harmless. When this occurs, the immune system will produce antibodies called immunoglobulin E (IgE) that trigger a chain reaction, causing white blood cells (like mast cells and basophils) to break open and release histamine and leukotriene into the bloodstream. These are the two chemicals that cause the main symptoms of allergy.

  • What are the common causes of a food allergy?

    It's thought that food allergies are caused by a mix of factors, including genetics, food exposures, and even hygiene. Eight foods account for 90% of all food allergies:

  • What can cause a skin allergy?

    A skin allergy in the form of a rash, hives, or angioedema may be caused by a substance you are exposed to internally (such as a drug, insect sting, or food) or externally (referred to as allergic contact dermatitis). It may also be the result of environmental stimuli such as extreme vibration, sudden temperature changes, or pressure placed on the skin.

  • What causes sun allergy?

    Sun allergy is caused by exposure to ultraviolet (UV) radiation. The allergy may occur on its own due to underlying genetics. Or it may be induced by medications such as retinoids or tetracycline antibiotics that cause photosensitivity. In some people, chemicals found in fragrances and disinfectants can instigate a rash when applied to the skin and exposed to sunlight.

  • What type of allergy causes puffy eyes, cheeks, and lips?

    Angioedema is an immune reaction that can cause generalized swelling of the eyes, cheeks, and lips. It can affect the hands, genitals, and other parts of the body as well. As opposed to a rash or hives, angioedema occurs in deeper layers of the skin, causing them to fill with fluid and form large, non-itchy welts.

  • What causes oral allergy syndrome?

    Oral allergy syndrome (OAS) is not a true allergy but one in which people with a pollen allergy will experience generally milder allergy symptoms when eating certain foods. OAS occurs when allergenic proteins in food and certain pollens are similar, triggering a cross-reactive reaction confined mainly to the lips and mouth. Examples of cross-reactive allergens are birch and apple, grass and celery, and ragweed and banana.

  • Can an allergy kill you?

    Yes, it is possible. If angioedema is severe and causes extreme swelling of the throat and windpipe, suffocation, and death can occur. A potentially life-threatening, whole-body allergy, known as anaphylaxis, can trigger an extreme drop in blood pressure, leading to shock, coma, multi-organ failure, and death. People with a history of severe allergy are typically advised to carry an epinephrine autoinjector (EpiPen) in case of emergency.

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