Help me doctor, I am allergic!

More and more people are allergic, children and adults. Our genes, as well as the environment and living conditions, will affect the occurrence of allergies. Could it be the disease of the XXIe century?

One of the most prevalent ailments of modern time is, once again, involved with our immune cells. The WHO (World Health Organization) itself ranks allergies, manifesting by a hypersensitivity, in fourth place of worldwide diseases. Allergy etymology comes from Greek allos : other and ergon : reaction. It implies a different reaction, often stronger, when a second encounter happens with the same compound. Hypersensitivities are linked to a tolerance lost regarding environment compounds, usually harmless, which induce a danger signal to the immune system in allergic people.

A panel of allergens

Allergens are compounds that induce allergic reactions. It varies in nature such as aeroallergens that we breathe in, trophallergens that we eat, transcutaneous allergens that touch our skin, drug related allergens, professional allergens but also bugs’ and reptiles’ venoms.

Aeroallergenes, also called airborne allergens include ever-present dust mites, pets’ hair, pollens, molds, gluten, cockroaches, and latex. It can be a bit surprising to find those last two in the list of airborne allergens, but cockroaches and latex produce airborne proteins in the environment that can become allergens.

Among trophallergens we can find child food allergies related to milk, eggs, peanuts, and gluten. Milk and eggs allergies are often temporary and are not found in adults. Concerning food allergies in adult are fruits allergies, vegetables allergies, seafood allergies and gluten.

It is important to notice that allergies and intolerances to food are different and have not the same mechanisms of action. Gluten intolerance is an auto-immune chronical gut disease which evolves during several years whereas gluten allergy is acute and happens almost immediately. Allergies are tricky since cross-over allergies can occur when someone is, for example, pollen allergic but also fruit allergic because both specific pollen and fruit happen to have a common allergen compound.

Figure 1 : Allergens ultimate trial. (Source : Adapted from Page 15 / Numéro 117 / Allergologie Pratique / Juin 2016)

Different types of allergies

An allergy is an immune system dysregulation that induces loss of tolerance for normally harmless compound, allergens. There are different forms of allergies which may be generalized, cutaneous (skin related) or respiratory (lungs related).

Generalized allergy is the anaphylactic shock which can be lethal if not treated fast. It can be triggered by food intake or bug bite that possess the allergen. It will induce a decrease in arterial pressure, swellings, respiratory inconveniences, diarrheas, awareness troubles and hemorrhagic syndrome.

Cutaneous allergies are allergic hives, eczemas, or dermatitis. They mainly appear as big red blotches that can become infected if the wound is open.

Respiratory allergies, rhinitis and asthma, will provoke respiratory troubles. For asthma, the patient won’t be able to let out the air within his lungs and will have choking sensation. Hay fever is an allergic rhinitis. The symptoms are conjunctivitis (irritated eyes) as well as a nasal airway obstruction and sneezes. It is important to bear in mind that an allergy form is not naturally linked to the allergen form. If the triggered allergy is asthma, it doesn’t have to be provoked by an airborne allergen. A food allergen could have induced asthma just as well.

Figure 2: Incidence of allergies during life. (Source: Adapted from Czarnowicki et al, 2017)

Factors influencing allergies

Many factors take part in development of allergies. One of the first is the genetic predisposition. Indeed, atopy, which means the ability to produce IgE antibodies against allergens, is a hereditary phenomenon. Yet, most of allergies are induced by IgE antibodies. A study led on penicillin allergy, a drug related allergy, brought to light genetic differences within some genes ruling the immunomodulation. Thus, genetic variations have been discovered for people having an atopic tendency. Some of these variations impact MHC genes (see article Immune system‘s G.I. in action) which is a protein involved in immune response.

Gender balance doesn’t exist in immune disorders since women are more affected than men. In fact, in vitro experiments have shown that testosterone was able to inhibit the maturation of some immune cells involved in inflammation and allergy. Furthermore, rodent studies demonstrated similar results since androgenous hormone receptors, male hormones, seems to play a protecting role against allergies.

Another hypothesis to explain the rise of allergies in past years is hygiene abuse. Increase of sanitation has diminished the risk of infections but has introduced a weakness in regard of allergies. A child too protected while young has less tolerance against allergens due to unconditioned immune system. Moreover, children born from caesarean section tend to develop allergies since they lack encounters with maternal bacteria. The same also applies to breast feeding versus baby milk formula in the early life of a child.

It is of utmost importance to bring a varied diet to children as a mean to teach immune cells tolerance. There is a strong link between microbiota and allergies. Food industry has been formatted to create processed and pasteurized products in the last decade which diminishes the bacteria existing within food who educate our immune cells. Tolerance to symbiotic bacteria, which exist naturally in the body can occur. Thanks to those bacteria, intestinal microbiota is taking shape. This last one is essential because the presence of commensal bacteria provides protection against other harmful pathogens (see article Immune system‘s G.I. in action).That tolerance induces a lymphocyte Treg enrichment in our body that will modulate and reduce the immune response. Probiotics and prebiotics have been introduced for that purpose in the last several years. We could see them as adjuvants helping to provoke an immune response (see article Vaccination: the bodyguard). They promote microbiota proliferation to control allergies by bringing a tolerance to modulate the immune response.

Figure 3: Factors involved in allergies. (Source: Adapted from Burbank et al 2017)

Global warming causes some tree species to grow in area that were previously inhospitable and pollen season are longer. Air pollution takes part greatly in allergy development. Stress, more and more present in our life, is also involved in allergy onset. Many external factors are able to modify gene expression without changing the DNA by mechanisms called epigenetic. Hence, someone’s environment and lifestyle greatly affect the occurrence of allergies. Allergies are part of our everyday lives and will be for a long time.

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