Allergy can manifest in many different ways. Different symptoms may appear at the same time, or at different times in a sufferer's life.

Epidemiological studies have been shown that the life of an allergic individual starts at a very young age.

If an infant is atopic, he/she has a risk of developing allergies which is linked to familial factors but also to exposure to allergens. He/she will start his allergic life with sensitisation to foods. These food allergies, which are very common in infants, then have a tendency to decrease or even disappear within the years. He/she may after be sensitised to inhaled allergens resulting in respiratory allergies such as asthma or rhinitis. Dust mites are responsible for 80% of allergic asthma in children. Thereafter, sensitisation to other allergies can occur in the individual. This is referred to as multiple sensitisations.

Figure 1: Outline of the natural history of childhood allergic manifestations as a function of age[1]

Allergies can affect different parts of the body: the nose, bronchial tubes, eyes, skin, and the gastrointestinal tract.

All of these targets have two special features:

1. They are organs in contact with the environment, and thus the allergen.
2. They have mast cells which participate in the allergic reaction. These are cells that have the job of warning our bodies about an environmental attack.

[1] Charpin D  et al. Epidémiologie des maladies allergiques respiratoires. In: Vervloet D, Magnan A. Traité d'allergologie 2003 ; Médecines Flammarion Ed Paris: 337-353.