What Actually Makes You Itch and Swell? One Cell Emptying Its Pockets
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
A cell stuffed with chemical granules sits in the tissues waiting to release them, and when it does the result is swelling, itching and, occasionally, collapse within minutes.
What the cell holds
The cell is packed with hundreds of granules containing ready-made chemicals, which is unusual, since most cells synthesise what they release when it is needed rather than storing it. Histamine is the best known of them and is responsible for much of the immediate effect, widening small blood vessels, making their walls leaky and stimulating nerve endings. Other granules hold enzymes that break down tissue, and substances that attract further immune cells. Because the contents are pre-made, release is essentially instant, which is what allows an allergic reaction to develop within seconds rather than hours.
Where they sit and why
The distribution is not random and explains where symptoms appear:
- •Concentrated in skin, airways, gut lining and around blood vessels
- •Positioned at the boundaries between the body and the outside
- •Long-lived, remaining in tissue for months rather than circulating
- •Coated with antibody molecules that act as triggers
- •Densest in exactly the tissues where allergic symptoms occur
- •Present in nearly all vertebrates, so the system is ancient
How the trigger works
The mechanism explains why a first exposure to an allergen causes nothing and a later one causes a reaction. On first exposure the immune system may produce a particular class of antibody against the substance, and those antibodies attach to the surface of these cells and sit there. On a later exposure the allergen binds to two adjacent antibodies at once, linking them, and that linking is the signal that causes the cell to empty its granules within seconds. The requirement for two to be bridged is why the reaction needs a molecule with at least two binding sites, and why very small molecules generally provoke allergy only when attached to something larger.
What happens in the severe case
When release happens across the whole body rather than locally, the consequences are immediate and dangerous. Blood vessels widen everywhere at once, which drops blood pressure sharply, while fluid leaking from those vessels into tissue swells the airway and can close it. Airway muscle contracts, making breathing harder still. The combination can kill within minutes and is why adrenaline is the treatment, since it narrows vessels, opens airways and stabilises the cells against further release, and why it must be given immediately rather than after antihistamines. Antihistamines block the receptors histamine acts on and do nothing about the rest of what the granules contain.
What the system is actually for
A mechanism this dangerous presumably exists for a reason, and the leading explanation is defence against parasites and venoms. The same antibody class is produced strongly against parasitic worms, and the resulting response, with fluid leaking into tissue, increased mucus, itching and gut contractions, is well suited to expelling a large parasite that cannot be engulfed by a cell. Evidence has also accumulated that the response degrades snake and bee venoms, with experiments showing mice lacking these cells are more susceptible to certain venoms. On that reading, allergy is a defensive system firing at a harmless target rather than a system with no purpose.
The takeaway
The cell stores hundreds of pre-made granules, so release is instant rather than requiring synthesis, which is why allergic reactions develop in seconds. Antibodies from a first exposure sit on its surface, and an allergen bridging two of them is the trigger. The cells concentrate in skin, airways and gut, which is where symptoms appear. The system appears to have evolved against parasites and venoms.