What causes allergies?
To help answer this question, let's look at a common household example. A few months after the new cat arrives in the house, dad begins to have itchy eyes and episodes of sneezing. One of the three children develops coughing and wheezing. The mom and the other two children experience no reaction whatsoever to the presence of the cat. How can we explain this?
The immune system is the body's organized defense mechanism against foreign invaders, particularly infections. Its job is to recognize and react to these foreign substances, which are called antigens. Antigens often lead to an immune response through the production of antibodies, which are protective proteins that are specifically targeted against particular antigens. These antibodies, or immunoglobulins (IgG, IgM, and IgA), are protective and help destroy a foreign particle by attaching to its surface, thereby making it easier for other immune cells to destroy it. The allergic person however, develops a specific type of antibody called immunoglobulin E, or IgE, in response to certain normally harmless foreign substances, such as cat dander. Other antigens, such as bacteria, do not lead to production of IgE, and therefore do not cause allergic reactions. IgE was discovered and named in 1967 by Kimishige and Teriko Ishizaka.
In the pet cat example, the dad and the youngest daughter developed IgE antibodies in large amounts that were targeted against the cat allergen. The dad and daughter are now sensitized or prone to develop allergic reactions on subsequent and repeated exposures to cat allergen. Typically, there is a period of "sensitization" ranging from days to years prior to an allergic reaction. Although it might occasionally appear that an allergic reaction has occurred on the first exposure to the allergen, there needs to be prior contact in order for the immune system to be poised to react in this way. It is important to realize that it is impossible to be allergic to something that an individual has truly never been exposed to before, though the first exposure may be subtle or unknown.
IgE is an antibody that all of us have in small amounts. Allergic individuals, however, generally produce IgE in larger quantities. Historically, this antibody is important in protecting us from parasites. During a sensitization period, cat dander IgE is overproduced and coats certain potentially explosive cells, such as mast cells and basophils that contain various mediators, such as histamine. These cells are capable of leading to an allergic reaction on subsequent exposures to allergen, which is the cat protein in this example. The cat protein is recognized by the IgE leading to activation of the cells, which leads to the release of various chemicals, including histamine. These chemicals, in turn, cause localized swelling, inflammation, itching, and mucus production, all of which are typical allergic symptoms. Once primed, or sensitized, the immune system is capable of mounting this exaggerated response with subsequent exposures to the allergen.
On exposure to cat dander, whereas the dad and daughter produce IgE, the mom and the other two children produce other classes of antibodies, which do not cause allergic reactions. In these nonallergic members of the family, the cat protein is eliminated uneventfully by the immune system and the cat has no effect on them.
Immune System + Foreign Substance (Bacteria, Viruses, Aeroallergens [Pollen, Animal Protiens, Dust Mites, Mold] Food, Medication)
|Normal Immune Response (Nonallergic and Allergic Individuals)||Exaggerated Immune Response (Allergic Individuals Only)|
|IgM, IgG, IgA, and various immune cells respond to exposure||IgE is overproduces in respones to aeroallergens and other harmless allergens|
|Foreign substance is eliminated.||Subsequent exposure results in allergic reaction.|