allergen
Meaning of allergen: Something harmless in itself that a particular immune system has learned to treat as a threat.
Definition of allergen
An allergen is a substance that provokes an allergic reaction in people sensitized to it, while causing nothing at all in everyone else. Most are proteins, and most are ordinary features of daily life: pollen, dust mite droppings, animal dander, mold spores, insect venom, foods such as peanut, egg and shellfish, and some medicines. The substance itself is not toxic.
Why does the body react to something harmless?
The first exposure produces no symptoms. Instead the immune system misreads the substance as a threat and makes an antibody called IgE against it, which then coats mast cells in the skin, airways and gut lining. That is sensitization. On a later encounter the allergen bridges two of these antibodies on a mast cell surface, and the cell dumps its contents within seconds. Histamine and related chemicals widen small blood vessels, make them leak, tighten the airways and irritate nerve endings. Itching, sneezing, swelling and wheeze all follow from that single burst. The allergic response is therefore a case of the immune response aimed at the wrong target.
Where do they come from?
| Group | Common examples |
|---|---|
| Airborne | Tree and grass pollen, dust mite, mold, animal dander |
| Food | Peanut, tree nut, milk, egg, wheat, soy, fish, shellfish |
| Venom | Bee and wasp stings |
| Contact | Nickel, latex, fragrance, some plants |
| Medicines | Penicillins and related antibiotics among others |
Why do reactions differ so much in severity?
Route and amount matter, and so does how widely the mast cells involved are distributed. Pollen meeting the nose lining produces local symptoms. A food protein absorbed from the gut, or venom injected under the skin, reaches the bloodstream and can set off mast cells throughout the body at once. That is anaphylaxis, in which blood pressure falls and the airway swells within minutes. The same allergen can cause a mild reaction on one occasion and a severe one on another, which is why past mildness does not predict the next episode. Not every unpleasant reaction to a food is allergic: lactose intolerance, for instance, is an enzyme shortage with no immune involvement.
How is one identified?
Two methods dominate. A skin prick test places a drop of purified extract on the forearm and pricks through it, and a raised itchy bump within about twenty minutes shows that IgE against that substance is present. A blood test can measure the same antibodies directly. Neither result proves that the substance causes symptoms, only that sensitization exists, so findings are read alongside the history of what actually happened and when.
Used in a sentence
Skin testing identified dust mite as the principal allergen driving the patient's symptoms.
Good to know
The severity of a past reaction does not predict the severity of the next one to the same allergen, and reactions confined to one site differ fundamentally from those that involve the whole circulation.
Word origin
Where “allergen” comes from
From the Greek allos, meaning other or changed, plus the ending -gen, meaning something that produces, so a producer of altered reactivity.