19 Mar, 2021
Sneezing, watery eyes, coughing, itching, breathing difficulties: every year the spring season inaugurates a series of annoying symptoms that unfortunately allergy sufferers know well. Allergic rhinitis alone is estimated to affect on average 23% of the European population.
What does it mean to be allergic? Why are some allergies more common than others? What precautions should observe the allergic subjects included in the Covid-19 vaccination campaign? We are answered by Dr. Samuele Burastero, Head of the cellular and molecular Allergology laboratory at the San Raffaele Hospital, and Dr. Mona-Rita Yacoub, Coordinator of the Allergology Area, Rheumatology, Immunology, Allergology and Rare Diseases Unit at the San Raffaele Hospital.
Allergy is an abnormal reaction against foreign substances that are otherwise completely harmless. The immune system of allergy sufferers “makes a mistake” and identifies a harmless substance, called an allergen, as if it were a dangerous microorganism. From this premise we understand how wrong it is to consider allergy a “weakness” of the immune system, since vice versa it represents an error due to excess response.
The term “antigen” indicates the components of viruses, bacteria or parasites that are correctly recognized by the mammalian immune system in order to defend the body from their respective aggressions. In people with allergies, even perfectly harmless substances, allergens, act as antigens. The characteristics which make an antigen an allergen are not completely known.
The amount and route of exposure can cause an antigen to become an allergen. For example, bakers may develop asthma or wheat flour contact dermatitis if genetically predisposed. Repeated administration of a substance may also be associated with an increased risk of sensitization. This occurs for example in the context of allergic reactions to drugs.
The genetic predisposition to the development of allergies, called atopy, is an essential factor that favors their onset. Continuous exposure to the allergen and for a given route, for example by inhalation, can cause allergy in genetically predisposed subjects. For example, a cat allergy cannot develop in the absolute absence of contact with the dandruff of these animals.
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These phenomena can be articulated differently in the various stages of life, depending on the environment in which the allergic patient lives, and also considering that with the years and the continuous exposure to a certain allergen, partial or total tolerance to the allergen can be established. In the context of drug allergy, the intermittent and repeated intake of a drug is one of the most important factors associated with the likelihood of developing an allergy to that particular drug, while continuous intake is a protective factor.
It is normal for the body to “get used” to a certain allergen, although this can take months, years or decades, depending on the subject, the type of exposure and the amount and frequency of introduction of the allergen (via food or by inhalation, for example). This process can be associated with more or less serious risks. The mechanism that presides over this phenomenon is immunological tolerance.
Tolerance to allergens can be induced in a controlled and safe way rather than by natural exposure, through the continuous administration of standardized extracts containing the allergens involved in the triggering of symptoms (so-called “allergen-specific immunotherapy”, improperly called “vaccine” against allergy).
Allergy is treated with two main categories of drugs, those that treat the symptoms (symptomatic) and those that treat the cause.
Symptoms are indicated in most cases, and are aimed at counteracting disorders in the organs in which they occur, for example the nose in the case of rhinitis, the eye in the case of conjunctivitis, the airways in the case of asthma. The most used drugs are antihistamines, which are taken by mouth and have an overall effect on both the nose and the eyes. Antihistamines have a good safety profile, but can induce sleepiness in variable ways from person to person. In asthmatics, on the other hand, inhaled corticosteroids and long-acting bronchodilators are used in the “background” therapy, i.e. the one to be taken daily during the faulty season (or all year round in the case of perennial asthma) to control the inflammation that characterizes asthma. In addition to this therapy, there are short-acting bronchodilators to be used as needed in the event of an asthma crisis.
The causal treatment of allergy is based instead on the administration of purified and standardized allergenic extracts, sublingually or subcutaneously, in order to induce immunological tolerance (allergen-specific immunotherapy). Once the effect has been established, it lasts for a few years after suspension. However, to achieve this result, immunotherapy must be continued for at least 3 years, which makes patients’ adherence to this treatment particularly low.
An unprecedented worldwide vaccination campaign is underway and some cases of Covid-19 vaccine anaphylaxis have led to severe concern in allergic patients. First of all, it should be specified that allergic reactions to vaccines represent rare adverse events, in particular if we consider serious ones such as anaphylaxis, described so far with an incidence of 2.5-11.1 cases per million doses administered of the Covid-19 vaccines currently used. In terms of comparison, anaphylaxis from parenteral or oral antibiotic class of penicillins occurs with an incidence of 1-4 in 10,000 to 1: 200,000 cases respectively.
Studies are underway to determine the cause of allergic reactions to Covid vaccines, although early data suggest the role of some excipients such as macrogol (polyethylene glycol, PEG) and polysorbates, rather than the antigens contained in vaccines. The Scientific Societies of Allergy and Clinical Immunology promptly proposed guidelines to protect the health of allergic patients and prevent them from excluding themselves from the vaccination campaign [1, 2].
To date, the indications are to subject patients with suspected allergy to these excipients to specific allergy tests since allergy to vaccine excipients is currently the only contraindication to vaccination. For allergy sufferers, prolonged post-vaccination clinical observation is recommended (from 30 to 60 minutes depending on the case). It is also essential to proceed with the vaccine in the phase of good control of asthma and chronic urticaria: it is important that patients with these pathologies who are not well controlled with the therapy in place are visited by their allergist or pulmonologist of reference for the appropriate therapeutic advice.
San Raffaele has a clinical allergy center and a research laboratory.
The clinical center is part of the national network for severe asthma (Severe Asthma Network Italy, SANI), has participated and participates in clinical trials with drugs for immunotherapy and with monoclonal antibodies that counteract the inflammation that characterizes the allergic response and TH2- broadly mediated in severe asthma, relapsing nasal polyposis, atopic dermatitis and chronic refractory urticaria. One of the two medicines for the treatment of grass allergy registered in Italy was approved with the contribution of our center. The clinical center is also highly specialized in the diagnosis of allergic reactions to drugs, including vaccines, and has a Day Hospital where the most complex tests are carried out.
The laboratory, focused on the study of the mechanisms of local immunity in the asthmatic lung, in recent years has been involved in the characterization of molecular allergens used in allergy diagnostics. Over the years, the allergology laboratory has developed second level diagnostic tests that are currently available for all patients thanks to their implementation in the clinical laboratory (ISAC microrarray, basophil degranulation test, determination of diamino-oxidase).
 SIAAIC – Guidelines for the management by allergists of patients at risk of allergic reactions to vaccines for Covid-19 http://www.siaaic.org/?p=5569
 AAITO – Guidelines for the management by allergists of patients at risk of allergic reactions to vaccines for Covid-19 https://www.aaiito.it/
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