What is an allergy?

Allergy definition

An allergy is a disorder caused by an abnormal reaction to a harmless substance called an allergen. The word allergen literally means substance capable of producing allergies. Allergens are tolerated without problems by non-allergic individuals. For example, most people tolerate being in contact with cats, but when you are allergic to them you start to sneeze, and you get an itchy and runny nose.
This allergic reaction is due to a “mistake” of the immune system. This system is responsible for overseeing the proper functioning of the body. It is similar to the security forces of a state (police, customs, military, firefighters, etc.). It protects us against invasion by harmful external agents such as bacteria or viruses that can cause infections, or against your own cells that might pose a threat, such as tumor cells. But it must also be able to allow the entry of beneficial substances as the nutrients ingested with food. When developing the allergy, there is a local inflammatory reaction or generalized symptoms against the allergen that should be tolerated and which will lead to development of symptoms.

Types of allergy

There are two main types of allergic reactions:

  1. Immediate hypersensitivity: this is what we usually call allergy. It is mediated by an antibody called immunoglobullin E (IgE), and it is fast in occurring (less than 30-60 min). This mechanism is related to disorders such as rhinitis or food allergy. In this case, skin tests to diagnose the allergy are performed by prick test, and the reading will be performed at 10-15 min.
  2. Delayed hypersensitivity: this reaction is mediated by some cells (T cells), and it occurs after several hours, usually 24-48 h. This mechanism is related to disorders such as contact eczema (allergic contact dermatitis). In this case, skin tests to diagnose the allergy are usually performed by patch testing (epicutaneous tests), and the reading will be performed after 2 to 4 days.

Another concept that is closely related to allergy is to atopy. Atopy is the tendency of an individual to develop allergic diseases such allergic asthma, allergic rhinitis or atopic dermatitis (eczema). For example, a child with asthma allergic to dust mites is an atopic child, and if his father also suffers from allergic asthma, we can say that this child has a family history of atopy. Atopy is an inherited tendency to allergy which is present in some families.

“Actors” of the allergic reaction

Actors of the allergic reaction

In the allergic reaction the following elements are involved:

  • Allergen. Substance, usually a protein, capable of generating an allergic reaction.
  • Immunoglobulin (IgE). Typical type of antibody involved in allergic reactions, which also participates in defense against parasites (eg, against worms).
  • Mast cell. Immune system cell which can be found in the respiratory mucosa, digestive tract and skin. IgE molecules are attached to their surface. Mediators are store inside mast cells, which are released during the allergic reaction inducing allergy symptoms.
  • Histamine. A substance that is stored within the mast cell and released during the allergic reaction. Is able to produce dilation of blood vessels (vasodilation), increased permeability of blood vessels (fluid leak) and nerve stimulation. This leads to the development of redness (erythema), swelling (edema) and itching (pruritus).

Allergic reaction

Allergic reaction

The allergic reaction takes place in two phases:

  1. Sensitization phase: In this phase, the immune system begins to “react ” against the allergen. I Antibodies against the allergen are produced. At this stage, the patient does not develop any symptoms.
  2. Symptomatic phase: In subsequent contacts of the patient with the allergen, the allergen binds to the IgE which is on the surface of mast cells. This binding works like a key fitting in a lock and “opens ” the mast cell. Substances that induce the typical symptoms of allergy are released. Among them is histamine, the most well-known mediator.