The predominance of the allergic illnesses has increased in the last 25 years, especially in infantile population and in young adults, and at present there are much more frequent such illnesses like the rinitis, the asthma or the dermatitis atópica.
It seems clear that the allergy development is determined by the interaction between the genetic predisposition and the exhibition to environmental factors.
For years it is known that a genetic predisposition exists for the "atopia", and they even have gone so far as to identify the genes of the allergy. The children with familiar allergy precedents in the first generation have more risk of developing allergic sensitization. When two parents present allergic problems, the risk that his progeny has them is 60 %, while if the only one of them endures the allergic illness, the risk is located concerning 25 %.

The exhibition to allergens (or proteins that give allergy) is indispensable for the allergy development, and both the age and the duration and the intensity of the exhibition influence also the sensitization risk. Once a patient has been sensitized to an allergen it has more risk to be made allergic to others.

Also, certain environmental factors protect or favor the allergy appearance in those patients with genetic predisposition.

Between the factors of risk proposed for the development of allergic sensitization they have been included:

• Season of birth: The allergy to pollen of birch or to pollen of gramíneas is more prevalente in children who are born in the spring, the children who are born in any other season. Of equal form, the children who are born in autumn and in winter in cold regions and coexist with domestic animals, such as cats, are sensitized more easily to these animals. All this suggests that the precocious and intense exhibition to certain allergens during the first months of life favors the allergy later to the same allergens. It seems clear that even if the allergy can take place at any age, it has big importance what happens in the first year of life.

• Exhibition to mites. The allergy to the domestic mites also has increased of substantial form in the last years. The allergens derived from the mites accumulate especially in the mattresses, pillows and couches, and the children who live in humid climates and who see a high number of hours of television, are sensitized more often to the domestic mites.

• Domestic animals: The allergy to proteins of the domestic environment is a lot of more frequent today than 30 years ago. They have contributed of decisive form certain changes to it in the life habits. For example, the favourite animals adoption (dogs, cats, etc.), introducing them in the housings and surrendering for it to direct and intense exhibition to the allergens that come from them.

• Plants and urban trees. Many of the plants that grow in the urban environment, exposed to high indexes of atmospheric pollutants, produce big quantities of proteins of stress (contained in the pollen), what determines that the persons who live in the urban environment are exhibited to pólenes with major concentration of allergens than those of the rural environment.

• Infections appellants: Any infection provokes a mucous inflammation, and the micoroorganismos can alter the immune answer and act like allergens, or at least like necessary co-factors. But recently a possible protective role has been suggested also opposite to the allergy of the infections endured in the first year of life.

• Contamination: One of the factors that more has fought like favoring of the development of allergic sensitization there are the particles derived from the combustion of the diesel engines. It seems clear that the exhibition to this type of pollutants alters the mechanisms involved in the answer IgE (or allergic).

• Tobacco: The smoke of the tobacco is, undoubtedly, the biggest pollutant of the interior of the housings. The exhibition to tobacco smoke collaborates of significant form with the appearance of allergic sensitization, with asthma and with other respiratory illnesses. The children exposed to smoke of tobacco present with more frequency allergy and asthma clinical, and to a great extent the nicotinism of the parents marks the prognosis of these children.

It results so from interest to identify those patients at the risk of developing allergic sensitization, with the object of adopting measurements of prevention that in many cases are simple, cheap and very effective.