How well do anti-histamines typically work at treating allergies?

Blocks hist. release: A person has allergies if his immune system over-reacts to certain types of pollen, and the histamine (and other chemicals) released by his immune system causes itching, runny nose, watery eyes, wheezing, cough, lots of mucus, etc... The treatment is avoidance of pollen, and use of antihistamines and nasal steroids. Antihistamines block the release of histamine from types of white blood cells.

Good for mild allerg: Antihistamines work pretty well for mild, intermittent allergies, or as a second medication in patients with more severe disease. For persistent symptoms that require daily medications, then nasal steroid sprays are superior.