Allergic diseases have become one of the major health problems of our days. Over the last 20 years the incidence and severity of allergy has increased 2- to 3-fold, fact that has prompted many scientists to characterize it as the new epidemic of the 21st century. In Europe and North America 25 % of adult population suffers from allergies. In children the situation is worse and allergic disorders have become the most common cause of chronic illness. In Europe 1 in 4 children is allergic and 1 in 7 children has allergic asthma. It has been predicted that if the increasing prevalence of allergies is not halted, by the year 2015 50% of Europeans will have an allergic disease. The rise of allergies has caused serious public health concerns. Moreover, the cost for allergy treatment is huge. In the European Union more than 29 billion euros per year are spent in allergic diseases.

What is allergy ? The word allergy is ancient Greek (allo + ergo) and signifies a reaction that is different from the appropriate. The Greek words anaphylaxis and atopy also indicate the aberrant allergic reactivity. Today it has been established that allergies are due to the development of an inappropriate immune responses against common, non-pathogenic, environmental substances. In every organism the immune system recognizes all the incoming substances and characterizes them as pathologic or not, in order to initiate the right immune response that provides protection and maintains the status of health. If the incoming substance is a microbe the immune system recognizes it as “pathogenic-dangerous” and induces a protective immune response that promotes the immune defence and eliminates the microbe. If the incoming substance is benign, e.g. pollen or house dust, the immune system recognizes it as “non-pathogenic” and does not initiate a strong immune reaction. At this point there is an immune malfunction in the individuals with allergic predisposition. In these individuals the immune system fails to recognize some naïve environmental proteins as such and initiates an unnecessary, non-protective immune response that leads to the development of allergic symptoms.

An allergic reaction can be triggered by a wide range of common substances, which are called allergens. Among the most common allergens are pollen, house dust mites, moulds, house animal dander (e.g. cat, dog etc), foods, drugs and insect venoms (bees, wasp). Someone can be allergic to one or multiple allergens. Long and repeated exposure to an allergen increases the chances of a predisposed individual to develop allergy. The type of allergen to which someone is sensitized influences the kind of allergic symptoms he develops. For example, inhaled allergens cause primarily symptoms from the nose or lungs, such as allergic rhinitis and asthma, while allergens that come into direct contact with the skin cause preferentially allergic dermatitis or urticaria. However, exposure to an allergen can also trigger symptoms from several organs (systemic reaction) irrespective of the route of allergen entry into the body. Usually allergies to foods, drugs or insect venoms present with multiple symptoms from the skin, the respiratory or the gastrointestinal system and occasionally the blood circulation. The systemic reactions are potentially the most dangerous.

	Doctor Daphne Tsitoura gives a lecture at the Onassis Foundation

Allergy is an old disease and descriptions of allergic symptoms exist in ancient Greek and roman texts. Nevertheless, significant progress in the elucidation of the mechanisms mediating the disease has been achieved only during the last 50 years. The revolution of new knowledge in the field of immunology has been mainly responsible for the improvement of our understanding of the immune process that leads from allergen exposure to disease manifestation. One of the cornerstones of modern allergology has been the discovery of immunoglobulin E (IgE) in the sixties. Today we know that allergic symptoms arise as a result of a chain of complex immune reactions activated following exposure to allergens. In particular, it has been demonstrated that in the predisposed individuals initial allergen exposure induces the aberrant activation of a particular type of  allergen-specific T helper lymphocytes (the immune cells that orchestrate the development of immune responses) that secrete soluble mediators, such as the interleukin 4 (IL-4), that enhance the synthesis of IgE antibodies. In our days allergen sensitization is defined by the presence of increased levels of allergen-specific IgE in the blood. IgE tends to bind to the surface of some special blood-born cells (the mast cells and basophils) that are plentiful in the skin, nose, lungs, eyes and gastrointestinal tract. The next time an allergic individual comes into contact with the specific allergen, the allergen gets captured by the IgE on the surface of these cells and allergic mediators, such as histamine, are released and produce within minutes the symptoms of early allergic reaction e.g. itch, edema, skin redness, sneeze, cough, running nose etc. This initial reaction is often followed by the recruitment of inflammatory cells to the site of allergen entry and the development of local inflammation. Continuous allergen exposure leads to chronic repetition of this cycle that eventually generates persistent inflammatory changes and manifestation of chronic disease, such as allergic rhinitis, asthma or eczema.

The development of allergic diseases has a genetic basis. Atopy indicates the genetic predisposition for enhanced synthesis of IgE and development of allergic symptoms. A lot of scientific interest has been focused in the identification of the genes that control asthma and allergies. However, the picture is not clear yet. The way allergic diseases are inherited is very complex and it seems that different genes play a different role in the various groups of patients. In general, it has estimated that the risk for a child with one allergic parent to develop allergies is approximately 25-30%. If both parents are allergic the risk rises to 60%. Nevertheless, allergies may “forget” one generation. This means that apart from the genetic predisposition other factors exert a deterministic influence. The environment and lifestyle play a crucial role. Novel epidemiological data show that the conditions prevailing in daily life, particularly during childhood, can positively or negatively modulate the factors that control the expression of allergy. In particular, the changes in the environment that result in reduced exposure to bacterial substances and the changes in infant nutrition are implicated in the sharp increase of the prevalence of allergic diseases in the westernized countries. According to this theory, known as the “hygiene hypothesis”, the rise of allergies is a consequence of the marked decrease of infections during the early years of life due to the improvement of sanitation in modern cities (clean water, clean food, clean homes) and the discovery of vaccines and antibiotics. Pathogens trigger in the body a series of strong immune defence mechanisms that eliminate infections and in parallel suppress allergic reactivity. Thus, early activation of natural immunity against infections provides some prophylaxis and reduces the chances for the development of severe allergic sensitization. It is still unclear whether the air pollution is a risk factor for the development of respiratory allergies. On the contrary, it has been confirmed that indoor pollution - particularly from smoking – has a direct correlation with the display of allergy symptoms.

To prevent the development of chronic allergic disorders early diagnosis and treatment of allergies is necessary. The family history is often a good indicator of an individual’s risk to develop allergies. A careful personal medical history and analysis of the clinical symptoms usually point the diagnosis. However, accurate assessment and confirmation of allergic sensitivity can be achieved only with allergy tests (skin or blood tests). If  the sensitivity to an allergen is determined it is very important to avoid or limit further exposure to this substance in order to inhibit or restrict the allergic reactivity. If allergy symptoms are already present, medication maybe necessary to control them and delay the development of chronic inflammation. There are several types of anti-allergy and asthma drugs available (e.g. antihistamines, corticosteroids, anti-leukotrienes, bronchodilators etc). Most of these drugs are quite effective in suppressing the symptoms and/or restricting the degree of inflammation. However, these drugs do not modulate allergic sensitization and therefore the relief is temporary and depends on the continuous use of medication. The only curative form of allergy treatment that is available today is the specific allergen immunotherapy (also known as allergy desensitization). Immunotherapy consists on the continuous administration of progressively increasing doses of the allergens to which the patient is allergic, in a way that promotes the development of immune tolerance to allergens. This results in loss of allergic sensitivity and lack of reactivity upon future exposure to the allergens in question. Immunotherapy has several restrictions and must be given only by well trained doctors.

In our days a great deal of research is taking place for the discovery of novel more effective forms of allergy and asthma therapy. To achieve this goal, academic institutes, pharmaceutical companies and international health organizations have intensify their efforts to inform the public, promote allergy research and support clinical allergology. We hope that the efforts will be fruitful and soon we will be able to celebrate significant advances in the understanding, prevention and treatment of allergies.