Allergies, also known as allergic diseases, are a number of conditions caused by hypersensitivity of the immune system to typically harmless substances in the environment. Allergy is an excessive response by the immune system to contact with an external substance considered harmful (allergen). Examples are pollen or the hair of certain animals.

Allergy indicates a condition in which the immune system of a subject reacts in an abnormal way producing antibodies against some substances, considered harmful, that for most people are harmless. The tissues and organs involved in allergic reactions are the lower airways, skin, nasal mucous membranes, eyes. Symptoms are due to the release by the body of chemical mediators (eg histamine) in response to the immune reaction triggered by the encounter of allergens with antibodies. The severity of allergies varies from person to person and can range from mild irritation to anaphylaxis (severe, potentially life-threatening allergic reaction).

The allergic reaction is a specific immune response mediated by class E immunoglobulins (IgE) produced by mast cells. In the allergic individual, these antibodies are produced when he comes into contact with the allergen to which he is sensitized and provoke an abnormal response, which induces the disorders typical of allergies.

The allergic process develops in two distinct phases:

  1. Sensitization: the immune system identifies the substance as an allergen. This phase takes place silently after the first contact. Macrophages identify the substance penetrated into the body and induce the reaction of lymphocytes, which produce specific IgE against the allergen. IgE enter the circulation and adhere to the membrane of mast cells, defense cells located in the skin, lungs and nose. Mast cells will have the task of identifying the allergen when it comes into contact with the body for the second time (immunological memory).
  2. The real allergic response, appears on the occasion of a subsequent contact with the allergen, giving rise to the typical manifestations of allergy. The IgE present on the membrane of mast cells recognize and capture the substance, causing the reaction of the immune system and the release of various active chemicals (histamine, leukotrienes and others). Prolonged or regular exposure to the allergen can cause the inflammatory response to become chronic and lead to disorders such as asthma.


The immune system has the task of protecting the body from the attack of harmful and potentially harmful elements, such as viruses and bacteria. Other foreign substances are normally harmless and, if they come into contact with the defense system, they are ignored and do not cause any problem. In predisposed people, the body triggers an allergic reaction of hypersensitivity towards specific allergens.

Among the triggering factors most responsible for allergic reactions are: airborne allergens (pollen, animal hair, dust mites and mold); some foods (eggs, milk, fish, shellfish, peanuts, tree nuts, wheat, soy); insect stings such as bees or wasps; drugs; substances that, in contact with the skin, irritate it (such as latex and nickel).

Allergens can be: inhaled with the breath, as is the case with pollen, animal dander, mold and dust mites; ingested, as is the case with foods and medications.Allergens can also penetrate through stings (as is the case with wasps or bees) or induce an allergy by coming into contact with the skin and mucous membranes, as is the case with metals and cosmetics.

The allergic reaction can manifest itself with different degrees of danger: from a simple hives to asthmatic pathology, up to the most serious form, the anaphylactic shock, characterized by reduction of blood pressure, increased heart rate, bronchoconstriction, up to the loss of consciousness. In the latter case, it is necessary to suddenly administer adrenaline, which being a strong cardiac stimulator favors the increase in pressure.


The best treatment for any allergy is to take appropriate measures to avoid exposure to the allergen. Sometimes, it is impossible to completely prevent any contact, however.

Medications are able to relieve allergy symptoms include:

  • Antihistamines: they help relieve symptoms such as itching, sneezing and nasal congestion and act by counteracting the formation of histamine, which is produced by the immune system and is active during the allergic reaction.
  • Local corticosteroids (nasal sprays): can reduce the symptoms associated with inflammation of the upper respiratory tract.
  • Decongestants: can be used for short periods to provide rapid relief of nasal congestion.
  • Antileukotrienes: block the action of certain immune system chemicals that cause certain symptoms, such as excess mucus formation and nasal congestion. These medications have also been shown to be effective in treating allergic asthma.

Allergic pathologies

The main diseases in which allergies come into play are contact dermatitis, transplant rejection, some forms of hypersensitivity to drugs and infectious agents, such as viruses, fungi, parasites. A third type of allergic reactions originates from the damaging action caused by antigen-antibody complexes (immune complexes) that are deposited inside the vessels and tissues and that, through the activation of the complement system, stimulate a local inflammatory reaction responsible for the damage. Examples of pathological situations in which this type of reaction plays a key role are allergic vasculitis, serum sickness, some glomerulonephritis and reactions to drugs. In this type of hypersensitivity allergens, as well as external (serum, drugs) can also be internal, ie structures belonging to the host organism, thus giving rise to an autoimmune process, as happens in diseases such as systemic lupus erythematosus and rheumatoid arthritis.

Great importance has also been given to food allergies, i.e. those abnormal reactions caused by the ingestion of food. Clinical symptoms are common to other types of allergies; they arise a few minutes or a few hours after the assumption of the food and manifest themselves with symptoms of different severity, which may consist of anaphylactic shock, asthma, rhinitis, hives, eczema, gastroenteropathies. The most incriminated foods are milk, eggs, fish, especially crustaceans, peanuts and American peanuts, strawberries and dried fruit. To remember cow’s milk, which is the most important allergen in early childhood, and allergies of infants to certain foods taken from the mother and carried by breast milk.

Intolerance to additives is confirmed only in 2 out of 10 thousand people: it is more frequent in children than in adults and can aggravate atopic dermatitis, chronic urticaria, rhinitis and asthma. The additives most responsible are tartrazine (a yellow dye), sodium glutamate (soy sauce and stock cubes), nitrates and nitrites (substances used in the preservation of cooked meats), sulfur agents (preservatives present in wine, in fruit juices, sausages, fruits and vegetables), benzoic acid and benzioates (antibacterial and anitimicotic preservatives for many foods), carmine (a red dye), vanillin, antioxidants and some enzymes, including papain. Finally, an allergic state can appear following exposure to physical agents, such as heat, cold and sunlight and is manifested by itching, hives and rashes, sometimes arising after bathing or swimming.

Considering the multiple causes that trigger them, in recent years allergies have become increasingly frequent and complex to diagnose and treat. Since the 1970s, allergies have increased by 30-50%. In general it can be considered that in European populations there is an annual increase in the frequency of allergies of at least 10-15%. This increase is observed for all allergens, whether they are inhaled (pollens, mites, animal derivatives, fungi, etc.), ingested (food and drugs), inhaled or stung (drugs, insect poisons) or contact (various chemicals, metals).

The occurrence of atopic dermatitis and clinical manifestations related to food allergy mainly privileges childhood, and then gradually decreases in old age. Bronchial asthma caused by environmental allergens, such as dermatophagoids (dust mites), frequently begins around the 3rd – 4th year of life, allergic rhinitis usually appears after the 5th year, while pollinosis begins later with a peak around 20 years of age for Parietaria and around 20-30 years for pollens of grasses and other herbaceous and tree plants.

Allergies also occur preferentially in males up to 15 years of age and in females at a later age. The significant increase in allergic diseases, documented by numerous epidemiological researches, seems to be caused by the increase in the number of circulating allergenic substances, many of which do not exist in nature (for example, chemicals, food additives, synthetic fibers); by the increased frequency of exposure to various allergens (consider for example the enormous consumption of drugs); by an increase in predisposing factors, including air pollutants. It also seems to go hand in hand with industrialization and civilization.

In fact, some studies have shown that developing nations have a low frequency of allergic diseases. Other studies have unexpectedly denied the responsibility of air pollution in the increase of allergies. In fact, the research that compared data from some cities of the former East Germany, with high pollution, with those of West German cities, with low pollution, has caused quite a stir. The results showed that in the latter the prevalence of allergies was higher than in the former. However, the high concentration of diesel exhaust particles is attributed to the ability to interfere with various immunological phenomena.

The link between exposure to cigarette smoke in the first months of life and the onset of allergies, including bronchial asthma, is also indisputable. Those who belong to a high social class are also more likely to develop allergies, as are those who are exposed to a high allergen load. The appearance of ragweed allergy in Lombardy, a phenomenon that was completely inconsistent until 15 years ago, is in fact linked to the impressive spread of this plant in recent years in the area north east of Milan. Another interesting hypothesis, supported by several researchers to justify the increase of allergies, considers that better hygienic conditions would favor a greater development of allergies.

The decrease in recent years of infectious diseases, which for thousands of years have engaged the body in the production of specific antibodies to eradicate them, would have caused a greater willingness of the immune system to sensitize to allergens by deviation from the normal anti-infective response to the anti-allergic one. The most recent data have also highlighted the increase in children living in industrialized countries, where cases of asthma due to allergies have doubled since the 1980s. Decreased maternal breast-feeding, environmental pollution, significant household allergen load (mites and pets), infections and diet during pregnancy are indicated as the main causes of the 30-50% increase in allergies during childhood.

According to some studies, carried out in Europe and the United States, the chronological order of birth of children in a family influences the prevalence of allergic diseases, and especially asthma. The last born in a family with more children are therefore less likely to suffer from allergic rhinitis, atopic dermatitis and asthma. Genetics also plays a recognized role. A child who has both parents allergic has a 60% risk of being allergic in turn. This risk is reduced to 40% if he or she has only one allergic parent.

Several investigations have also indicated that the month of birth of the child may be important for subsequent sensitization to pollens, a hypothesis that finds its rationale in the immunological situation “in progress” of the child during the first 30 days of life. Other surveys carried out in various European countries have shown that on average 25% of school-age children have one or more allergic diseases.

In recent years, in parallel with the increase in sales of cosmetic products, there has been a progressive increase in the incidence of contact dermatitis from cosmetics, which currently represents 15% of contact dermatitis in general and as much as 28% of extra-professional contact dermatitis, thus ranking second after allergies due to metal accessories. It is therefore estimated that they affect about 2% of the population, mainly women between 20 and 40 years old. Among the triggering factors are indicated the composition of cosmetics, the concentration of ingredients, the place of application, the contact time, the frequency of applications and the cumulative effects of preservatives, fragrances, excipients and emulsifiers.

There are also allergies to Hymenoptera venoms: hypersensitivity to particular venomous secretions of these insects (bees, hornets, wasps) may cause a modest local reaction or a ponchus, usually itchy and often painful. Rarely, anaphylactic shock may occur.

  • Allergies of the respiratory system
    • Allergic asthma
    • Allergic rhinitis
  • Cross allergies
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