An allergy is an immune system disorder that occurs when an individual develops an abnormal sensitivity to an environmental substance. Substances that are known to cause the development of an allergy are called allergens (in contrast to antigens, which is the term describing all substances capable of provoking an immune response, whether allergic or not). Common allergens include pollen, insect venom, pet dander, dust mite feces, penicillin, egg white, peanuts, and wheat gluten.
An allergy is the result of hypersensitivity to an allergen. Hypersensivity means that the immune response is highly reactive to an allergen even though the allergen is not harmful. This hypersensitive response is thought to develop when, during an initial exposure to a particular allergen, the immune system mistakenly produces large amounts of reactive antibodies.
The type of antibody usually produced in an allergic response is called IgE, or Immunoglobulin Type E. This antibody is heavily involved in the development of inflammation. Each subsequent exposure to the allergen can cause mass production of IgE, causing an allergic response which can include respiratory symptoms, skin reactions, and other problems, depending on the nature of the allergen.
What Happens during an Allergic Reaction?
When an allergic individual is exposed to an allergen they are sensitive to, several things happen in very quick succession – in very sensitive people, the reaction is virtually immediate, and in some cases, potentially fatal if not treated quickly.
The release of IgE triggered by an allergen causes immune cells called granulocytes to release molecules that trigger an inflammatory response. This response is extremely rapid, and in some cases occurs immediately upon exposure to an allergen.
Symptoms of allergy include swelling, itching, and redness at the site of contact, and an increase in mucus secretions. Depending on the site of contact, these symptoms can be life-threatening. If they occur in the lungs, for example, these symptoms can cause breathing difficulty. Other symptoms, such as involuntary muscle contractions, nerve stimulation, and blood vessel dilation, can become serious if not treated. Severe swelling can also be fatal due to contraction of airways.
Some allergy symptoms are local (confined to one area of the body), whereas others are systemic, and occur all over the body. The nature of the allergen and the exposure to it, along with the degree of individual sensitivity to the allergen, is generally what determines whether symptoms are local or systemic. For example, asthma causes local symptoms confined to the lungs, whereas a penicillin allergy can cause systemic symptoms.
Theories on Causes of Allergies
Currently the true cause of the allergic reaction is not fully understood. Most research points towards the existence of a genetic basis for allergic sensitivity. Twin studies, for example, show that identical twins are more likely to share the same types of allergies than are fraternal twins. Most people will also notice that allergic sensitivity does tend to run in families.
However, the two individuals in an identical pair do not always develop an identical allergy pattern, which means that genetics don't tell the full story. It's likely that environmental factors are important too.
On the basis of twin studies in asthma, it has been determined that the tendency to develop asthma is inheritable, rather than the condition itself being inheritable. It’s probable that the same is true of other types of allergies. What this means is that if you have close relatives with certain types of allergies, you're likely to be more susceptible to developing those types of allergies, if the environmental conditions that trigger the allergies are present.
Another interesting fact is that the overall incidence of allergy has increased rapidly within a relatively short space of time – too short to be fully explained by genetic changes in the population.
So what might have caused this? One theory is that an increased emphasis on hygiene in the modern world has led to a decrease in exposure of children to infections. According to this theory, reduced exposure to common childhood infections means that people are more likely to develop inappropriate immune responses to substances which the body would normally see as harmless.
Asthma and Allergy Foundation of America: What Causes Allergies? <https://www.aafa.org/display.cfm?id=9&cont=79>
Los H, Postmus PE, Boomsma DI. Asthma genetics and intermediate phenotypes: a review from twin studies. Twin Res. 2001 Apr;4(2):81-93.
The Mayo Clinic: Food Allergy Causes <https://www.mayoclinic.com/health/food-allergy/DS00082/DSECTION=causes>