Having a hypersensitivity means that someone’s immune system has reacted to something in such a way that it ends up damaging them, as opposed to protecting them.
There are four different types of hypersensitivities, and in the first type or type one, the reactions rely on Immunoglobulin E, or IgE antibody, which is a specific type of antibody - the other major ones being IgG, IgA, IgM, and IgD.
So because IgE is involved with type one hypersensitivity reactions they are also called IgE-mediated hypersensitivities.
This type of reaction is also sometimes called immediate hypersensitivities, because the reaction happens super fast—on the order of minutes.
So most allergic reactions are IgE-mediated, and therefore most allergies are type I hypersensitivity reactions.
“Allergy” comes from the Greek Allos which roughly means “other” and ergon which means “reactivity”.
Essentially, allergies are reactions to molecules from outside your own body that most people don’t react to—and these are specific molecules from things you might breathe or take in like foods, animal dander, bee stings, mold, drugs or medications, and pollen.
You can also mount an allergic reaction to things you come in contact with on your skin like latex, lotions, and soaps.
These specific molecules are also called antigens, and when they cause an allergic reaction, they’re called allergens.
An allergic reaction happens in two steps, a first exposure, or sensitization, and then a subsequent exposure, which is when it gets a lot more serious.