In an allergic person, the mast cells of the nasal lining are already studded with IgE against some allergen, their granules preloaded with histamine and waiting.In an allergic person, the immune system has already made IgE antibodies against some allergen. These IgE sit like little antennae, studded on the mast cells of the nasal lining.
The mast cell is packed with granules preloaded with histamine and other signal molecules on standby. For now all is quiet — the system is merely idling, waiting for an enemy it has decided to watch for.
2 · The allergen cross-links two IgE
When the allergen enters the nose again, one allergen grabs two adjacent IgE at once, and this cross-link is the fire signal the mast cell receives.The next time this allergen (say, pollen) drifts into the nose, the key step happens: one allergen grabs two adjacent IgE at once, building a bridge between them — this is called cross-linking.
Note: it takes two IgE bridged together to trigger it; a single occupied IgE doesn't count. It is exactly this bridge that becomes the fire signal the mast cell receives.
3 · Degranulation · histamine bursts out
Once the cross-link signal arrives, the mast cell degranulates within minutes and dumps its preloaded histamine and other mediators into the surrounding tissue.The moment the cross-link signal enters, the mast cell degranulates: the preloaded granules rush to the membrane within minutes, rupture, and dump histamine and other mediators into the surrounding tissue.
It is an instant burst of ammunition prepared long in advance — nothing has to be made on the spot, which is why an allergic reaction comes on so fast and so hard.
4 · Symptoms · immunity misfiring
Histamine dilates blood vessels, makes glands secrete and nerves itch, bringing sneezing and a runny nose: immunity has mistaken something harmless for an enemy, it is not too weak.As histamine spreads, it makes blood vessels dilate, glands secrete, and nerves itch — so within minutes come the sneezing fits, watery runny nose, and itchy eyes and nose.
The counterintuitive key: this whole thing is the immune system mistaking a harmless thing for an enemy — it is over-active and misdirected, not too weak. That is why antihistamines help (they block the histamine step), while trying to treat allergy by boosting immunity points exactly the wrong way — what you want is to help this misfiring system calm down, not step on the gas.