Causes and Risk Factors of Allergies

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.

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.

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.

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

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

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.

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

• Bee stings (honeybee, wasp, hornet, yellow jacket)
• Mosquito bites
• Fire ant stings
• Bed bug bites

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

• 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.

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 doctor, 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.

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 reason. 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.

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 doctor. If you have children or plan to have children, talk to your doctor 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.