Allergy can be described as a malfunction of the human immune system causing a violent reaction against normally harmless substances in our natural environment. In this context, such substances are generally referred to as "allergens".
All of the risk factors for allergy are not known, but genetic and environmental factors are of importance. People with a tendency to suffer from allergic conditions are said to be atopic.
Allergy can be antibody or cell-mediated. In the majority of cases, the antibody responsible for an allergic reaction belongs to the IgE isotype and individuals may be referred to as suffering from an IgE-mediated allergy.
Symptoms of allergy vary in complexity, severity and manifestation. With atopic allergies the effect of exposure to an allergen can be immediate. Symptoms can be mild or severe, varying from itchy eyes, to eczema, rhinitis, conjunctivitis, bronchoconstriction, vomiting and diarrhoea and, in rare cases, anaphylaxis. Chronic reactivities can be seen as skin reactions (eczema) and in bronchial asthma an ongoing lung tissue inflammation can be the reason for an asthma attack seen after exposure to the offending allergen. Asthma attacks can also be brought about by events other than IgE-mediated reactions.
Manifestations of atopic disease vary considerably depending on the allergen involved, as well as the age of the allergic patients. During infancy, allergies to food (especially egg and milk) are the most common. After the age of 3, most allergic children tolerate these foods, while allergy to inhalants becomes predominant. Specific IgE antibodies often precede allergy symptoms, but clinical symptoms catch up with time.
Physicians should investigate all allergy symptoms and the possible cause of the symptoms.
In its early stages, allergy can appear as a seemingly benign condition, such as hay fever during the pollen season. Some people gradually build up an allergic condition over several years before experiencing any symptoms. Others experience sudden, life-threatening anaphylactic shock following ingestion of a certain food or drug, or an insect bite. Additionally, some allergies that start as eczema or gastrointestinal problems can evolve into asthma, frequently involving respiratory symptoms such as hyper reactivity and obstruction of the airways.
Specific IgE blood tests can be performed irrespective of a patient’s age, skin condition, antihistamine medication and even during pregnancy. IgE antibodies appear in human serum and plasma as a result of sensitisation to a specific allergen. Measurement of circulating IgE provides an objective measurement of the sensitisation to the allergen.
Phadia, now Thermo Fisher Scientific, offers over 650 different allergens and 70 allergen components for sensitive and specific quantitative detection of allergen-specific IgE antibodies.
Although a final diagnosis should always be based on a combination of IgE test results and a patient’s clinical history, a general rule of thumb is that the higher the IgE antibody value, the greater the likelihood of symptoms appearing.
For an example on how quantitative test results can help determine if a child has a good prognosis to outgrow a food allergy, see Case Study Felix.