An allergy is a specific reaction of the body to certain, otherwise harmless substances from our environment (so-called allergens). The immune system recognises allergens as foreign substances and responds to them like to a pathogen by triggering an immune response. In contrast to the normal immune response, however, an allergy is clearly excessive and exaggerated.
By way of mediation by different cells, special allergen-related antibodies are produced. These antibodies react with certain structures in the skin and on the mucous membranes and trigger the release of messenger substances (hormones). The most important allergy hormone is histamine. It is responsible for many allergic reactions of the body, such as itching, constriction of the airways, or increased permeability of blood vessels.
An allergy causes disagreeable effects (runny eyes, runny nose, breathlessness, diarrhoea and skin reactions such as rash, erythema and itching), in particular on the mucous membranes of eyes, nose, bronchi and intestines as well as the skin.
Which substances cause an allergy?
Basically, every substance from the environment can cause an allergy: Plants (e.g. grass pollen), animal products (e.g. bee poison, animal hair, skin scales), but also metals (e.g. nickel) or chemicals (e.g. preservatives) can cause an allergy. After one allergic reaction of the immune system, it remembers that. So another allergic reaction develops very quickly upon repeated contact with the respective allergen.
What types of allergies are there?
- Inhalation allergy – inhalation of allergens (pollen, dust mite waste, mildew, animal hair...)
- Food allergy – reactions to certain food ingredients (nuts, seafood, apples, chicken protein…)
- Drug allergy – reactions to certain pharmaceuticals (for example, penicillin)
- Insect poison allergy – after insect bites (e.g. bee or wasp sting)
- Contact allergy – after skin contact with allergens (e.g. nickel or cosmetics, animal hair)
What manifestations of allergies are particularly common?
Allergic rhinitis (hay fever) is a common type of allergy to the proteins of certain pollen of grass, trees, herbs and dust mites. The allergy mostly manifests itself on the mucous membranes of the respiratory tract, the nose and the eyes.
Atopic dermatitis/neurodermatitis is a chronic eczematous disease of the skin caused by an increased reaction to external and internal irritants (constitutional). It manifests itself in the form of an inflammation. Patients often suffer from allergic asthma and/or hay fever. In addition, many patients have food allergies which cause skin reactions.
Allergic asthma develops in dependence on certain triggers (e.g. animal hair, dust mite waste, tree or grass pollen, mildew…). In the course of the allergic reaction, the bronchial mucosa swells up. More mucus is produced and the airways constrict. Typical symptoms are coughing fits, breathlessness, and chest tightness.
By nettle rash (urticaria) or hives, doctors understand a skin rash that develops suddenly and is most often associated with severe itching or stinging, and feels/looks like after contact with stinging nettles. A nettle rash can be caused by a variety of irritants (allergic, inflammatory or physical irritants).
Medical specialists (allergists) differentiate between 4 different types of allergies
Type I allergies (immediate-type allergies) are very common and are most often triggered by protein components. Clinical examples are hay fever, asthma, food allergies and insect poison allergies. The most severe manifestation of an allergic reaction of type I is the allergic shock (anaphylactic shock) which most often affects several organ systems at the same time and can lead to death in only a few minutes if left untreated, which is only rarely the case, however.
A type II allergy (allergy of the cytotoxic type) is a very rare type of allergy which can, for example, damage blood cells. Possible triggers of type II allergies can be pharmaceuticals.
A type III allergy is caused by antibodies which bind components dissolved in the blood. That results in larger aggregates which form on the vascular walls and can cause an inflammatory reaction there. On the skin, the allergy manifests itself in the form of an allergic skin reaction (skin rash, exanthema), as small, itching, dot-like red spots on the skin.
A type IV allergy (late-type allergy) is the only type of allergy where sensitised antibodies directly attack allergens. Typical type IV allergens are metals or chemicals. Clinically, a type IV allergy manifests itself as a contact eczema (erythema, itching, skin rash).
When and how does an allergy manifest itself?
The symptoms of an allergy persist as long as the allergen is present in the body. The type of allergy symptoms does not depend on the allergen, but on the underlying allergy type (type I, II, III or IV). The allergy type also determines the time of occurrence of the first symptoms of an allergy. They can develop either immediately (immediate reaction), hours later (delayed immediate-type reaction) or even days later (late reaction). Physical reactions of an allergy may cause topical (local) as well as generalised discomfort.
Local symptoms of an allergy manifest themselves mostly in spots that got in contact with the allergen, for example:
- the mucous membranes of the respiratory tract with sneezing, a cold, or breathlessness,
- the mucous membranes of the eyes with runny or itching eyes,
- the mucous membranes of the gastro-intestinal tract with vomiting or diarrhoea, or
- the skin with itching, erythema, hives, or skin rashes.
General symptoms of an allergy may manifest themselves all over the body if the substance that triggers the allergy is distributed in the organism through the blood (for example, in case of an allergy to bee or wasp stings). The cardiovascular system is particularly affected as the heart rate increases and the blood pressure drops.
Breathlessness, severe circulatory problems, tachycardia and skin rash can be symptoms of an allergic shock. Always call an emergency doctor when you notice possible signs.
How is an allergy diagnosed?
An allergy is diagnosed by an allergist.
The physician-patient talk serves as a basis for further tests. The physician will find out whether and what allergies there are in the family (parents, siblings, children) or manifested themselves in the patient in the past. An option for narrowing down the causes is the keeping of an allergy diary.
Depending on the cause and the physician, different tests are used (prick test, intradermal test, scratch test and rubbing test). Samples of possible allergens are applied to, scratched or injected in the skin in the process depending on the test method. When there is a positive reaction, pustules or wheals develop in that spot. In addition, a lab can test a blood sample for the existence of special allergy antibodies.
How is an allergy treated?
The best method for preventing an allergy is the so-called "allergen avoidance", which means that any contact with an allergen is avoided. However, that is only possible in rare cases. Classic examples are animal hair allergies and food allergies.
Hyposensitisation for allergies
The specific immune therapy (SIT), also referred to as hyposensitisation, aims at influencing the response of the body to the cause of the allergy in a targeted manner. The patient is administered slowly increasing quantities of the allergen. The immune system can get used to the substance and so learns to tolerate the exogenous allergen (e.g. bee or wasp poison).
Symptomatic therapy for allergies
Most allergies are treated with pharmaceuticals which alleviate or prevent the development of allergy-related symptoms, but cannot cure the allergy. Depending on the type and severity of the complaints, these so-called "antihistamines" are taken in different administration forms (tablets, nasal sprays, asthma sprays, eye drops, creams, ointments and injections) and at different intervals (acutely, prophylactically, continuously).
Active agents used for the systematic therapy of allergies are
- Antihistamines, e.g. diphenhydramine, loratadine
- Mast cell stabilisers, e.g. cromoglicic acid
- β2-sympathomimetics, e.g. salbutamol
- Leucotriene receptor antagonists for asthma, e.g. montelukast
- Theophyllin for asthma
- Immunomodulators (pimecrolimus and tacrolimus) for atopic dermatitis
- Immunosuppressants for very severe cases (cyclosporin A)
- The humanised monoclonal antibody omalizumab for severe allergic asthma
Severe acute cases of allergy (such as an anaphylactic shock) are life-threatening and require emergency medical care. Patients with known, life-threatening allergic reactions (e.g. insect poison allergies) can be prescribed an adrenal injector which they should have with themselves for an emergency.
List of references
Please note: This text is for your general information. Please do not make a diagnosis yourself, but consult a doctor in the event of questions or complaints. He/she is the only one to assess the symptoms correctly based on his/her clinical experience and to take further diagnostic steps as necessary.