The action mechanism of the allergic reaction

An allergic reaction is the result of an inappropriate immune response by the body after exposure to a foreign substance (the allergen).

The mechanism of allergic reactions is broken down into two successive phases: a sensitisation phase during which the immune system identifies the substance as an allergen.Then when the organism comes into contact with the allergen again, the allergic phase is triggered.

The allergy can affect the eyes, the skin and the airways such as the nose or the bronchi.

The most frequent mechanism, which causes rhinitis, conjunctivitis and allergic asthma, is that of immediate hypersensitivity.

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• When the allergen comes into contact for the first time with the body (for example, after it has been inhaled or swallowed), it is recognised by a special type of white blood cell: the macrophages. These cells are found in large numbers in the skin and in mucous membranes, and are part of the front-line immune defence of the body.
• These macrophages pass on information about this first contact of the body with the allergen to another type of white blood cell: the B cells. These B lymphocytes are then transformed into plasma cells which produce large amounts of the type of antibody which is ultimately responsible for the allergic response, Type E immunoglobulin (IgE). These IgE molecules are specific for a given allergen and are only found in subjects who are allergic to that particular substance.
• The IgE antibodies quickly find their way into the bloodstream and bind to cells called mast cells which are found in the skin and the mucous membranes. They contain a large number of granules and have the job of capturing the allergen when it next comes into contact with the body.
• At the same time, the "memory" of this first contact between the allergen and the body will be stored for an extremely long time (up to several years) by another group of immune cells called "Memory" T lymphocytes. This first phase is silent, that is to say, the individual undergoing sensitisation to an allergen does not feel any particular symptoms. The allergic symptoms (such as a runny nose, red, watery eyes, difficulty breathing, redness and itching of the skin) are not triggered until the next contact is made, even if it is after a very long period of time.

• During the second contact between the allergen and the "sensitised" individual, the IgE antibodies bound to mast cells now get involved. They directly capture the allergen, which results in mast cell degranulation (the release of small granules containing chemical substances). Among other things, these granules contain histamine, which plays a key role in the allergic response, since it is responsible for the symptoms (rhinitis, asthma, urticaria, etc.).
• Information about this second contact is propagated throughout the body, which leads to amplification of the allergic phenomenon.