Sensitization
Sensitization to an allergen typically begins with the first exposure of the human body to the allergen. When the allergen gets into the human body, it causes the immune system to react. Specifically, the allergen stimulates a type of immune cell called a Th2 immune cell. This kind of cell secretes a chemical called interleukin-4. Once this Th2 cell is stimulated and begins secreting interleukin-4, it causes certain other cells (called B cells) to become activated. These B cells then make antibodies (which are also known as IgE). These IgE molecules are designed to specifically bind to the allergen, and they float around until they come into contact with other immune cells (particularly mast cells and basohpils). Once they find these immune cells, they cover them, which causes the immune system to be "sensitized" to the allergen.
Genetic Theory
Allergies (and the sensitization to an allergen) are often the result of an abnormally functioning immune system. This is because the typical allergen does not threaten the human body in a specific way, which should normally cause it to be ignored by the immune system (and in particular the Th2 cells). It is not entirely clear what causes the Th2 immune cells to react to these allergens, although genetics are thought to play a part. Parents with allergies are more likely to have children with allergies, although the specific allergen may not be inherited (parents who are allergic to peanuts may have children with strong pollen allergies, for example), which suggests that the genetics have to do with the immune system, not allergen processing. Allergies are also most common in young children and some allergies will fade as the child gets older.
Hygiene Hypothesis
One theory about the physiology of allergies is also known as the "hygiene hypothesis". This theory is based on the fact that exposure to bacteria and other dangerous pathogens causes stimulation of a different kind of immune cell, called Th1. When these Th1 cells are stimulated, they suppress the activity of Th2 cells. Because Th2 cell activity is related to allergies, the hygiene hypothesis theorizes that living in a too sterile environment can cause inappropriate Th2 activation, which can lead to the development of allergies.
