Notice that not one person is covering their mouth
and nose here. Makes for better video,
A sneeze is the body’s way of trying to expel foreign material that is irritating the respiratory system, most likely the very upper respiratory system – the nasal passages. In the case of spring allergies, the offender is most likely to be grains of pollen. The rhinitis (rhino = nose, and -itis = inflammation) caused by seasonal allergens (hay fever to you and me) are small particles that stimulate an immune response for some reason. In almost all cases they are not harmful particles, as is the case with pollen grains, except that some can induce very strong allergic responses. So why do some people’s bodies try so hard to expel them?
An allergen is nothing more than a protein or carbohydrate, some sort of biomolecule, that your body recognizes as foreign and against which it mounts a specific immune response. For most people, any one specific particle may be seen as foreign, but your body doesn’t go crazy over it; it has been tolerized (learned not to respond), or the response is held in check by other parts of the immune system. But for those unlucky few (or many), pollen grains are a type of allergen that stimulates a large IgE (one type of antibody) response, along with chemicals like histamine and leukotrienes.
The reasons that some people develop an allergic rhinitis to one or more materials aren’t completely known. There is some evidence that if you are fighting off a viral infection and at the same time are first exposed to the allergen, then the heightened activity of the immune system will stimulate a response to the innocuous material. And once that happens, you’re sunk. The body has an immune memory; it builds a small army of cells that then recognize that particular allergen, and if it enters the body again, a strong response will follow and an additional memory response will be built.
It may be that too little exposure to bacteria and viruses (which stimulate more a type of immune response called Th1) actually makes the body more likely to go overboard when an antigen stimulates a Th2 response (the type of response induced by allergens). There needs to be a balance between Th1 and Th2 that helps keep them both from over-reacting. It is also possible that when babies are very young, before they have had time to develop their own adaptive immune system (build on their own by being exposed), it is important to stimulate their innate immune system (always on guard and doesn’t require learning to react). The innate response helps build the adaptive system and works to balance the adaptive Th1 and Th2 responses.
In terms of seasonal allergic rhinitis, the allergens we are talking about most often are pollen grains. Many plants are fertilized by insects; the insect comes to a flower to drink nectar, the pollen sticks to them, and when they get to the next flower, the pollen is transferred to the stigma and the male gamete cells grow out of the pollen grain via the pollen tubules, down to the ova and fertilized the egg. However, that isn’t the only way pollen grains can be dispersed to other plants; the wind often plays a role. Wind-pollinated plants have small pollen grains, light enough that they will be spread far and wide by a gentle breeze. Unfortunately, this is rather hit or miss; they aren't going to be blown directly to another plant of the same species (as a bee would carry them to the next flower). Since the chances of a single pollen grain finding a flower are low, the plant has to make millions of times more pollen grains. That is a problem for people who suffer seasonal allergies.
Iguanas, especially marine iguanas, sneeze more than any
other animal. The sneezing is a way for the to expel certain
salts that are a byproduct of their digestive process.
Immune cells are always on the prowl for foreign invaders, especially in/on parts of the body that contact the outside world. Your nose qualifies as such, so there are many immune cells patrolling your nasal passages that recognize specific antigens. When an immune cell meets that one antigen (or maybe two if there is a cross reaction) that it is built to recognize, it triggers a response. In the case of allergic rhinitis, the responses are to release an antibody type called IgE. The IgE then binds to other immune cells, like eosinophils and mast cells, and then release histamine and leukotrienes, amongst other things. The histamine makes your nose and eyes itch. The chemicals make the small blood vessels leaky, so fluid comes out making your eyes water and your nose run. They stimulate more mucus production, so you get congested. Blech! In addition, the histmaine and leukotrienes do one more thing, they stimulate nerve endings in your nose to trigger a sneeze. The sneeze is meant to get those allergens out of your system as quickly and forcefully as possible.
There is another reason why a sneeze might be in order during allergy time. A 2012 study showed that the mechanism to get rid of mucus (called the muciliary elevator) sometimes get stalled when mucus is overproduced and full of particles. The clearance mechanism uses the rhythmic beating of cilia on the nasal cells to brush the mucus toward the mouth to be coughed out or swallowed. The researchers used some nasal tissue and sent a pressure wave over the cells to mimic a sneeze. The pressure wave stimulated the cells to start clearing mucus by beating their cilia, so the scientists describe a sneeze as a rebooting of the mucus clearing mechanism. Unfortunately, people with chronic sinusitis and chronic allergies have nasal passage tissues that don’t reboot, so they just keep sneezing and sneezing without any relief. In the case of people with allergies, antihistamines and decongestants are a savior. For everyone else, just sneeze and be done with it – don’t self-medicate at the drop of a hat, people take too many drugs.
sneeze in response to sudden onset of a bright light. This is a genetic trait and involves higher brain centers, like the visual cortex. Therefore, it is a reflex that extends beyond the brainstem or spinal cord – very weird. It is called, for obvious reasons, ACHOOs (Autosomal Dominant Compelling Helio-Opththalmic Outburst syndrome). Other people suffer from snatiation –sneezing when their bellies are full. This is also genetic and is inherited as an autosomal dominant trait. And some people have a tendency to sneeze after being intimate. The weirdest? Sneezing with hoppy beers – but that’s another story.
Mark E. Lasbury, MS, MSEd, PhD
As Many Exceptions As Rules
Mark E. Lasbury, MS, MSEd, PhD
As Many Exceptions As Rules