Allergy can be viewed as an abnormal reaction of an individual's immune system towards a foreign substance (allergen) to which the same individual has been previously sensitized.
There are many kinds of allergens of different origin (animal, vegetable, chemical), such as moulds, pollens, foods, drugs, chemical molecules, and animal-derived substances.
The clinical manifestations of allergy will differ according to the way a person comes in contact with an allergen to which he or she has been sensitized (i.e. inhalation, ingestion, direct contact), and may vary from urticaria to rhinoconjunctivitis and asthma.
The factors leading to the establishment of an allergic response are very complex and include the interactions of genetic (family history), environmental, and immunological (humoral and cellular) factors.
From an immunological standpoint, a key role is played by the class E immunoglobulins (IgE) that are produced by allergic individuals in response to an allergic stimulus in large amounts compared to not allergic individuals.
Allergic symptoms are mainly due to the release of chemical inflammatory mediators, the most important of which are histamine and leukotrienes, which determine, for example, in hay fever, an inflammation of the conjunctiva and the mucous membranes of the nasal cavity and respiratory tract, resulting in redness, itching, swelling, tearing, and coughing.
In most cases, the allergic manifestations do not have serious consequences; however, they can adversely affect the quality of life and ability to work of affected individuals.
After an adequate diagnostic workup, the approach to the allergic patient may involve the use of medications such as antihistamines or steroids to alleviate symptoms.