allergy

Synonyms for allergy

Hypersensitivity, hypersensitivity

Definition of allergy

Allergy ( allo = altered, ergos = activity) refers to an overreaction of the immune system to so-called environmental antigens that are not normally pathogenic and are tolerated by the body. These antigens are also called allergens and describe certain smallest substances or their parts that activate the immune system.

Cross-allergy means the simultaneous existence of multiple allergies due to the structural similarities of the antigens. For example, an allergic person to pollen often also reacts sensitively to pome fruit.

Epidemiology

The allergy usually occurs in childhood, but can manifest itself again at any age. The allergy to food frequently occurs in the first months and at the age of 15 and 35 years.

The most common allergy is 60% against pollen, followed by 15% all mite allergies and allergy to animals and less than 5% allergies to drugs and food.

In Western countries, there has been an increase in the frequency of allergies for about 30 years. In Germany, for example, allergic hay fever currently affects 15% of the population, 5% suffer from bronchial asthma, 1.5% from a food allergy. Up to 20% of all German schoolchildren suffer from eczema ( atopic dermatitis ).

Causes of allergy

The causes of an allergy include the genes. If at least one parent suffers from an allergy, the risk for the children to develop such an allergy increases as well.

In addition, atopy should be mentioned in this context. Atopy is the inherited willingness of the body to form antibodies to normal antigens (IgE). As a result, there are allergic diseases, including bronchial asthma, eczema (atopic dermatitis), hay fever (allergic rhinitis and conjunctivitis ), skin wheals ( hives, urticaria ) and food and drug allergies.

The so-called hygiene hypothesis is also discussed as the cause. This indicates that in recent years, an increase in allergy is recorded because people grow up too sterile and come into contact with too little germs in childhood. As a result, the immune system can not develop properly and therefore is allergic to normal environmental antigens.

Common allergens include pollen, dust mites, animal hair, food (eg eggs, milk, nuts, soy, wheat) and medications (eg antibiotics ).

Pathogenesis of allergy

An allergy can only occur if a first contact with the triggering substance ( antigen ) and a subsequent sensitization have taken place. After first contact, a reaction of the immune system takes place, which does not show any external symptoms, that is, the patient does not notice anything of an allergy. This is followed by the sensitization phase, which lasts for 1 to 3 weeks and which also takes place in the immune system. There are special proteins ( antibodies ) that are directed against the antigen and sensitized cells ( lymphocytes ). As a rule, the patient also does not notice anything of this reaction, but this makes the antigen an allergen. This means that a renewed contact with this antigen now causes a visible reaction with symptoms of allergy and discomfort in the patient.

This visible reaction of Allegie can be assigned to a reaction type according to Gell and Coombs . There are a total of 4 types:

  • Type 1 allergy = IgE-mediated immediate reaction (IgE = antibody, immunoglobulin E)
  • Type 2 allergy = antibody-mediated cytotoxic reaction (IgG, IgM)
  • Type 3 allergy = immune complex-mediated reaction (IgG)
  • Type 4 allergy = cell-mediated reaction

These types have differences in response time after contact with the allergen, the affected organs, and the mechanism of action.

The types 1 to 3 of the allergy are mediated by antibodies, they are called humoral reactions. In contrast, type 4 is mediated by cells, which is why it is defined as cell-mediated.

Type 1 allergic reaction is most common and can be found, for example, in hay fever or bronchial asthma. Triggering allergens include pollen, mites in bed, animal hair, food or medicine. After contact with them, the allergic reaction occurs within 30 minutes on the skin, mucous membranes, respiratory tract or in the digestive tract. As a consequence of a release of certain substances ( histamine, serotonin, leukotrienes, prostaglandins ) from special cells of the immune system, the mast cells, typical symptoms such as runny nose, itching, etc. are referred to as mast cell degranulation. It expires when the allergens bind to certain antibodies, namely IgE. These IgE are located on the surface of the mast cells. Ig is immunoglobulins, E is the class.

The procedure described characterizes the so-called early phase or acute-phase reaction of this type of reaction; After 3 to 8 hours, there is also a late phase followed by inflammation and damage to the tissue, which can last several days.

For example, type 2 allergy occurs in some form of anemia (hemolytic anemia). In this case, the reaction occurs only after 5 to 8 hours. Allergens are mostly medications and affected organs are the blood cells or the kidney. The mechanism of action is characterized by the destruction of cells (lysis). Responsible for this lysis are formed antibodies against certain structures of the cell surface. The antibodies include IgG and IgM, ie class G and M immunoglobulins.

The type 3 allergy reaction takes place on the skin or systemically after 2 to 8 hours after allergen contact (molds, bacteria, medicines, etc.). In this way, very many complexes of allergens and antibodies directed against them are formed in the blood in a short time. These deposit as a result of the rapidly occurring amount in the tissue. This occurs, for example, in an inflammation of the renal corpuscles (glomerulonephritis) or in the case of serum sickness.

The longest time with 24 to 72 hours to react requires the type 4 reaction of allergy. This is visible, for example, on the skin, liver, kidneys or lungs. Among the allergens triggering nickel and other metals, drugs, disinfectants or cosmetics. A typical disease is contact dermatitis, which is an inflammatory change of the skin (eczema). The reaction is initiated by specially sensitized immune cells (T cells) by activating other immune cells (macrophages = scavenger cells, natural killer cells), which in turn damage other cells.

therapy

Allergy can first be treated by omission of the allergen, ie by avoidance and reduction of the triggering substances or allergens or by discontinuation of causative drugs.

Since this is not always or only with difficulty depending on the allergen, the therapy can be supported in certain diseases with the help of medication. It is possible to stabilize the mast cells ( cromoglicic acid ), to prevent the action of some of their released mediators (antihistamines, leukotriene antagonists) or to suppress the inflammatory reaction (steroids).

Furthermore, it is possible to perform hyposensitization in allergies to pollen, bee venom, house dust mites or mold fungi. As a rule, the corresponding allergen is usually injected under increasing pressure over the skin for 3 years in order to ensure that the immune system tolerates the allergen and no longer shows the allergic reaction.

In about 75% of patients, this treatment is successful. The exact mode of action is not clear.

Anaphylactic shock as an acute emergency is treated with fluid and oxygen administration, cortisone, respiratory extension drugs (? -Mimetics) and catecholamines (adrenaline, dopamine). As a last resort in respiratory or circulatory arrest resuscitation is used.

Prophylaxis of an allergy

Prophylactic of an allergy are breastfeeding the children and growing up in rural environment.

However, if there is already an allergy, triggering the allergic reaction by avoiding the allergen ( allergen avoidance) can be prevented.

Summary

An allergy is the hypersensitivity reactions of the immune system to allergens found in the environment. They are mainly visible on the skin, eyes, respiratory tract and intestine, for example itching, redness, wheals, shortness of breath or diarrhea.

Allergies are treated primarily by avoiding the triggering allergens. There are also drugs or hyposensitization.

What Are Food Allergies and How Are They Treated? (December 2019).


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