When we breathe, not only air enters our body. In the air are a lot of tiny particles. Some of the particles are smoke or dust that may carry allergens like dustmites. Some are actually bacteria that could make you sick (or may have already). Some others are chemicals like perfumes. Our nose has tiny hairs inside it to help filter particles like this out, but they can't always catch everything. When these particles hit the cells on the inner surface of your nose (called the 'nasal mucosa'), the cells respond by producing a type of chemical called a 'histamine.' Histamines can travel in between cells to reach the nerve cells in your nose. This is why many medicines that stop sneezing are called 'anti-histamines.' The medicines are actually chemicals that prevent histamines from reaching your nerve cells.
The nerve cells respond to the histamines by sending a signal to your brain, telling it that something is irritating your nose. From there, your brain sends a signal through other nerve cells that run into muscles all through your head and chest, making you sneeze. When you sneeze, you send air out of your body very very fast, pushing the irritating particles out with it.
Normally, the particles that your nose cells respond to are ones that really need to be removed from your body (like bacteria cells when you have a cold). Sometimes, though, the cells in your nose can respond to something totally harmless as if it were bad for you. This is what happens when someone has allergies. Things like dust mites, which are perfectly safe to breathe in, cause the same reaction as the things that are actually dangerous.
-Tamara & Adam
(published on 10/22/2007)