Another neat little factoid for you: allergies are caused because your body thinks it’s been infected with a parasite (think worms or some other gross intruders).
Normally how it works: a special type of immune system cell, called a Th2 Helper Cell gets activated for some reason by a parasite’s proteins, and it then activates B cells that then produce IgE antibodies. (Antibodies capture materials to keep them from harming cells and tell other cells that there’s some invasion in your body.) Those antibodies bind to anti-parasite cells like mast cells, who are basically just sitting around, waiting to dump tons of chemicals around cells to hurt the parasites, or at least flush them out. The mast cells can irritate the parasites, but their chemicals also cause muscle to contract to dislodge the parasites or cause your body to secrete more mucus to make it hard for the parasites to hold onto body tissues.
And that’s how it works for allergies, too. Antibodies capture allergens (like pollen, mold, ragweed), and mast cells tell other cells to step up secretions–there’s a parasite around! That’s why you get a runny nose and watery eyes–your body’s trying to flush out what it thinks is a bunch of parasites in your nose, eyes, mouth, and sinuses.