What is histamine?

Histamine is an immune response compound produced by the body that causes, among other things, inflammation.
When something that the body identifies as a threat comes along, some of the cells in the nearby tissue produce histamine in an attempt to remove the invaders.
In addition to inflammation and itching, fluid is released from tiny blood vessels as part of the allergic response, hence the runny nose and watery eyes commonly associated with hay fever.

What is antihistamine?

Basically, antihistamine (also known as histamine antagonist) prevents the production of histamine by introducing a competing response that blocks histamine from attaching to nerves, small muscles, mast cells etc which can bring reduce inflammation and mucous production
Itchiness and sneezing is prevented by antihistamine blocking your nasal nerve H1-receptors..

Antihistamine Nasal Sprays

Antihistamine direct to the nose is still relatively unknown on the hay fever scene in many countries.

Sources:

    1. AZEP Nasal spray Australian product information. MEDA Pharmaceuticals, January 2014.