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, especially when the cat comes into her bedroom. 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 are substances that are capable of causing the production of antibodies. Antigens may or may not lead to an allergic reaction. Allergens are certain antigens that cause an allergic reaction and the production of IgE.
The aim of the immune system is to mobilize its forces at the site of invasion and destroy the enemy. One of the ways it does this is to create protective proteins called antibodies that are specifically targeted against particular foreign substances. These antibodies, or immunoglobulins (IgG, IgM, IgA, IgD), 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. To summarize, immunoglobulins are a group of protein molecules that act as antibodies. There are five different types; IgA, IgM, IgG, IgD, and IgE. IgE is the allergy antibody.
(In 1967, the husband and wife team of Kimishige and Teriko Ishizaka detected a previously unrecognized type of immunoglobulin in allergic people. They called it gamma E globulin or IgE.)