The result of histamine release is leakage of fluid from blood vessels into the tissue and stimulation of nerve endings that gives a sensation of itch or tickle. The fluid leakage causes swelling of the tissue and accounts for the stuffy, runny nose which is one of the primary symptoms of allergy. The itch causes sneezing.
Antihistamines work by blocking the effect of histamine.
The stuffiness, which takes a little longer to respond, can also resolve. This is because the leaked fluid can be reabsorbed by the body and the antihistamine prevents further swelling. These medicines are often used, either alone or in combination with a decongestant, for the treatment of cold symptoms. Some people find that antihistamines reduce the runny nose of a cold and help the tickle. Since colds aren't caused by allergy, however, the beneficial effect of antihistamines for the cold is less than for allergy.