What exactly is allergy?

A allergy is a immune system reaction In front of one external substance which is usually safe for most people.

Although allergy is often described as an overreaction of the immune system, allergies are not defined by the intensity of the response, but rather by that abnormal response.

In this article we are going to define what an allergy is from a pathophysiological point of view and we will see the differences with other hypersensitivity reactions.

allergy definition

Clemens Freiherr, the first to describe allergies
Clemens von Pirquet, an Austrian scientist, was the first to use the word "allergy"

It is very common for the terms allergy and hypersensitivity reaction to be used as synonyms, even in specialized media, often defining allergy as an exacerbated or exaggerated reaction of the immune system.

However, one allergy is characterized by being a abnormal response In front of one usually harmless foreign substancenot by the intensity of the immune response, which can be variable.

The hypersensitivity reactionon the other hand, refers to the immune mechanism that causes allergybut there are other hypersensitivity reactions that are not allergic.

For example, the autoimmunity it is a hypersensitivity reaction against an endogenous antigen of the individual and its manifestations, as well as the response produced by the immune system, are very different from those of an allergy.

In this sense, allergy is defined as a type of hypersensitivity reaction. Specifically as a type I hypersensitivity reaction in the Coombs and Gell classification.

Immunological mechanism of allergies

Type I allergies or hypersensitivity reactions are mediated by IgE type antibodies Y inflammatory mediatorsmainly histaminereleased by mast cells and basophils, a type of immune cells.

Allergies require a first exposure to allergen, the substance that causes the allergy. In that first exhibition the immune system is sensitized and produces antibodies against the allergen, but symptoms do not usually develop.

In subsequent exposures, the immune system already recognizes the substance and will react quickly, so allergy symptoms appear quickly after contact with allergic substances.

Mechanism of an allergic reaction
Mechanism of an allergic or anaphylactic reaction

Allergens do not produce immune responses in most people, so it is understood that the origin of allergic diseases is in the individual and not in the substance that produces it.

The predisposition to be allergic it has a high environmental load and also hereditary. This predisposition is called atopy.

Allergic diseases include many forms of rhinitis, dermatitis, anaphylaxis, food allergies and some forms of asthma.

The symptoms can be very variable, but in all it occurs tissue damage in areas where the immune system mistakenly tries to defend itself.

Another very common confusion occurs with food allergies and intolerances. Food allergies are allergic reactions to food allergens, while intolerances are the difficulty in digesting certain nutrients, generally due to an enzyme deficiency, and have nothing to do with the immune system.

Types of hypersensitivity reactions

As we have seen, allergies are a type of hypersensitivity reactions, but there are other non-allergic hypersensitivity reactions.

In all of them, reactions of the immune system take place against substances that should not, but with the origin of the substance and the immune mechanism They are different.

There are several classifications of hypersensitivity reactions, one of the most used is Coombs and Gell classification. In this classification, allergies correspond to type I hypersensitivity reaction.

  • Type I hypersensitivity reaction: also known as immediate, anaphylactic or atopic reactions. The immune response is mediated by IgE-type antibodies and the release of histamine along with other inflammatory mediators by mast cells and basophils.
  • Type II hypersensitivity reaction: also known as antibody-dependent hypersensitivity reactions. They are mediated by IgM or IgG, the complement system, and antigen-presenting cells. Antibodies bind to antigens, both internal and external, but adsorbed on the surface of self cells. Ex: myasthenia gravis, autoimmune hemolytic anemia, erythroblastosis fetalis, etc.
  • Type III hypersensitivity reaction: also known as immune complex disease. It is mediated by IgG-type antibodies and the complement system, but the reaction is against soluble immune complexes, not against cell surface antigens such as type II. Ex: serum sickness, systemic lupus erythematosus, etc.
  • Type IV hypersensitivity reaction: also known as delayed hypersensitivity reaction, although type III reactions are also late reactions and should not be confused. It is mediated by T lymphocytes, monocytes and macrophages. Ex: some forms of contact dermatitis or multiple sclerosis.

Some authors add a fifth type, the type V hypersensitivity reactionwhich is an autoimmune and antibody-dependent reaction, similar to type II hypersensitivity reaction, but in which antibodies bind to cell surface receptors instead of surface antigens.

Grave's disease or myasthenia gravis could be considered as type V hypersensitivity reactions.


In summary, all hypersensitivity reactions are characterized by an abnormal reaction of the immune system, the allergy a specific type of these reactions, the type I hypersensitivity reactionthat is characterized by:

  • The immune system reacts against allergensexternal substances that do not usually cause reactions in most people.
  • This IgE-mediated and the release of inflammatory mediators by mast cells and basophilsanaphylactic reaction).
  • Your symptoms appear immediate or in a very short period of time after exposure to the allergen.
  • The symptoms are due to inflammatory reactions with local tissue damage, which can be generalized to produce anaphylactic shock.
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