What is allergy?

 

DEFINITION OF ALLERGY

Abnormal and specific reaction of the body in contact with a foreign substance (allergen) that does not cause trouble in most subjects.

   For an allergy to develop, it is essential that a first contact has taken place between the allergen and the subject's body. It is this sensitization that is abnormal, because the allergic manifestation that follows is only the consequence.

MECHANISM

The role of the immune system is to recognize antigens on the surface of particles or microorganisms and, in response, to produce antibodies (immunoglobulins) and white blood cells (lymphocytes). These will come into contact with these antigens to destroy the microorganisms that carry them. A similar process is triggered in the mechanism of allergy, but the immune system then produces antibodies and lymphocytes sensitized to harmless substances, allergens, which are considered particular antigens and, as such, dangerous. But this is not the case: it is the reaction to these antigens that is particular and possibly dangerous.

DIFFERENT TYPES OF ALLERGY

The inappropriate or exaggerated reactions that occur in allergies are called hypersensitivity reactions.

- Type I hypersensitivity, or immediate hypersensitivity, is the most common. The allergens in question come from pollen (grasses, trees, herbs), mites contained in house dust and pet dander (tiny particles of skin or hair), atmospheric molds, certain drugs (eg penicillin ) and food (eggs, milk, fish, crustaceans, dried fruits), insect venom (bee, wasp), food coloring (tartrazine). Some reactions following the ingestion of strawberries are not necessarily allergy but are due to a substance that promotes the release of histamine, without there being any antibodies against this substance. When in contact with one of these allergens, the elaborated antibodies, immunoglobulins E (IgE) bind to mast cells, skin cells and mucous membranes whose granules contain histamine (the substance responsible for the symptoms). inflammation and which also causes contraction of the bronchial muscles). In a second contact, the allergen unites with IgE, which leads to degranulation of mast cells, that is to say the release of histamine and, by that, the various symptoms of allergy: rash, itching, swelling, runny nose, nasal obstruction, sneezing, spasmodic cough, conjunctivitis, respiratory difficulty, diarrhea. This results in manifestations such as asthma, hay fever, urticaria, angioedema, anaphylactic shock and many types of food allergy.

- Type II hypersensitivity, which is less frequent, is of a cytotoxic nature; it involves IgG and IgM antibodies and is responsible for transfusion reactions, maternal-fetal incompatibility and haemolytic anemias. This reaction against other presentations of the same antigen is called alloimmunization. Some drugs may bind to an antigen (blood group for example) and turn it into an artificial alloantigen.

- Type III hypersensitivity involves IgG antibodies. It is responsible for glomerulonephritis, systemic lupus erythematosus and some form of lung disease, hypersensitivity pneumonitis (extrinsic allergic alveolitis, farmer's lung, bird breeder disease), as well as swelling of the skin during vaccination reminders. These antibodies bind to their antigens to form immune complexes which, when they are deposited, will cause the lesions of the disease.

- Type IV hypersensitivity is a delayed cellular reaction responsible for allergic dermatitis resulting from skin contact with certain materials (nickel, rubber, detergents, cosmetics) and granulomas (tuberculosis, sarcoidosis), as well as the rejection of a grafted organ.

CAUSES

Allergies occur in genetically predisposed individuals: allergic manifestations as diverse as asthma, atopic eczema, allergic rhinitis, or urticaria frequently affect many members of the same family. The mode of transmission of this predisposition is complex because there is not only one gene involved. Other factors promote the development of allergic reactions: the environment (presence of an animal or pollen), viral infections and emotional factors.

DIAGNOSTIC

Evoked from the personal and family history of the subject, his lifestyle, clinical signs (eczema, urticaria, conjunctivitis, hay fever, rhinitis, asthma, diarrhea, etc.), the diagnosis is supported by skin tests, and, in some cases, confirmed by laboratory tests.

SYMPTOMS AND SIGNS

They are those of any allergy: rash, asthmatic reaction, or even anaphylactic shock.

TREATMENT

A food allergy is treated in the same way as any allergic phenomenon. Prevention is the absolute exclusion of food from the responsible food.