In a person with allergies, the body has become hypersensitive to harmless substances, including pollen and grass, so the immune system must treat them as if they are pathogens
The front of the eye, much like the outer layer of the skin, protects us from environmental harm. When potentially harmful substances come into contact with our eyes, our response is to blink so that we can wash away these perpetrators. But this process isn’t foolproof, as anyone who has had something irritating in their eye can attest.
If the offending irritant is something that you are allergic to, then a whole host of biochemical processes can occur. Unfortunately for you, immune cells in your eye called mast cells are on the lookout for allergens, which are molecules that you are allergic to. The mast cell is a main player. Mast cells come from the bone marrow and are sent to places such as the eye as part of the first defense mechanism against invading pathogens.
Allergy in the eye is called allergic conjunctivitis, and it can range from mild to serious. When an allergen comes into contact with the eye, it binds to the antigen molecules on the surface of the mast cells. Mast cells are quick to react, releasing a number of chemical messengers, such as histamine, in a process that scientists call degranulation.
Histamine levels peak in your tears around 5 minutes after initial exposure to the allergen, and symptoms are naturally alleviated after around 30 minutes providing the allergen has been removed from the environment.
However, continued exposure can make your itchy eyes return. This is because there is often a second spike in histamine release sometime between 6 and 72 hours after exposure. A host of immune cells are implicated in this process, and histamine is once again released to haunt you and cause you to have itchy eyes.