When the allergy season strikes, you may get that familiar feeling of itchy eyes. But for many, it is more than just a nuisance – so what is the culprit causing you such distress?
It’s histamine. You might already know this if you are taking antihistamine-containing medication for your allergies, which just so happen to be the number one cause of itchy eyes. But what does histamine actually do?
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.
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.
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.
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 then activates specific itch nerve cells by binding to histamine receptors on the surface of these cells.
Scientists are still unsure about how exactly this works, but much progress has been made in recent years to further our understanding of the mechanism and biological purpose of itch.
What is known today about the science of itch comes mostly from studies on skin. Here, the purpose of itch is thought to be to encourage us to scratch, and, by doing so, dislodge any potential parasites that have attached themselves to our skin.
Once itch nerve cells in the eye have been stimulated by histamine, they send signals to the brain along the spinal cord. Your brain then tells you that your eye is itching.
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 some time 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.