histamine
Meaning of histamine: Four different receptors explain how one small molecule can cause hives, drive stomach acid, and keep the brain awake at once.
Definition of histamine
Histamine is a small signaling molecule made from the amino acid histidine and stored in granules inside mast cells and basophils. Released within seconds of an allergic trigger, it widens small blood vessels and makes their walls leaky, so fluid escapes into the tissue and produces swelling, redness and itch. It also works as a messenger in the stomach and the brain.
Why does one molecule do so many things?
| Receptor | Where it acts | What blocking it is used for |
|---|---|---|
| H1 | Small blood vessels, nerve endings, airways and brain | Hay fever, hives, itch and motion sickness |
| H2 | The acid-producing cells lining the stomach | Reducing stomach acid |
| H3 | Nerve endings within the brain | Disorders of wakefulness |
| H4 | Immune cells | Still under investigation |
The molecule is identical everywhere, so the effect depends entirely on which receptor a tissue carries. That is why a tablet for hay fever does nothing for stomach acid and why a medicine that lowers acid will not settle a rash. Both are antihistamines in the strict sense, and the everyday use of the word for the allergy tablets alone is a shorthand rather than a definition.
Why do some antihistamines cause drowsiness?
Histamine is one of the brain's wakefulness signals, acting there as a neurotransmitter released from a small cluster of cells in the hypothalamus. The older antihistamines are small and fat-soluble enough to cross the blood brain barrier, so they block that signal along with the one in the skin, which is where their sedative effect comes from and why several are sold as sleep aids. Newer ones were designed not to cross, relieving the same symptoms while leaving the brain alone. The same crossing explains why older antihistamines work against motion sickness and the newer ones do not.
What releases it?
The classic route is an allergic response, in which IgE antibodies sitting on the surface of mast cells are bridged by the substance they recognize and the cell empties its granules within seconds. Mast cells can also be triggered directly, with no antibody involved, by certain medicines, contrast dye, pressure on the skin, heat or cold, producing an identical rash through a different mechanism. Histamine can even arrive ready-made in food: fish left warm accumulates it as bacteria convert the histidine in the flesh, and eating it causes flushing, headache and a rash that imitates an allergy without any immune reaction taking place. In allergy skin testing a drop of histamine is applied deliberately as the positive control, to confirm that the skin is capable of reacting at all.
Used in a sentence
Histamine was used during allergy testing to confirm a normal skin response.
Good to know
Histamine acts on more than one type of receptor, which is why the medicines that block its effect on allergy-related receptors are chemically distinct from those that block its effect on the acid-producing cells of the stomach and are not interchangeable.
Word origin
Where “histamine” comes from
Formed from Greek histos, meaning tissue, combined with amine, a chemical class name, reflecting histamine's presence and effects throughout body tissue.