Pollen grains have a secret ingredient that maximizes their sneeze-producing potential, according to a new study.
Although the act of sneezing is fairly simple, the process that brings it about is anything but. When pollen enters the nose, immune cells called mast cells bind to a protein, or antigen, in the pollen grain and release histamine--a chemical that causes inflammation, which leads to itching and sneezing.
But this isn't the whole story, according to a new study published today in the Journal of Clinical Investigation. When academic allergist Sanjiv Sur at the University of Texas in Galveston and colleagues exposed mice to ragweed pollen, they noticed the animals produced highly-reactive forms of oxygen in their lungs after 15 minutes. These so-called reactive oxygen species (ROS) have long been known to be part of the allergic response, but they were thought to take 4 to 6 hours to show up. The team also found ROS in pollen-exposed mice that lacked the cells needed for ROS production.
So the team reasoned that there must be something in the pollen grains themselves that allowed the body to produce ROS so quickly. Sur knew that, in plants, an enzyme called NADPH oxidase makes the ROS necessary for root hair growth. On the hunch that the enzyme might also be involved in pollen tube growth, the team searched for NADPH oxidase in pollen grains. Indeed, they found the enzyme in 39 of the 40 types of pollen they surveyed. The final proof was in the pollen. When the researchers exposed mice to just the pollen antigen or to NADPH oxidase alone, the animals exhibited only minor symptoms. But when the team combined the antigen with the enzyme, the mice had a full-blown allergic response.
As a result, Sur and colleagues propose that pollen causes sneezing via a 1-2 punch: The NADPH oxidase in the pollen grain makes ROS, while the antigen leads to the histamine response. This means the pollen grain contains everything it needs to prompt an allergic response, says Sur. "Now we think of [the grains] as armed," he says. "They are charged with their own arsenal."
It's an elegant study says Marc Rothenberg, director of the division of Allergy and Immunology at Cincinnati Children's Hospital Medical Center. "It proved their theory beyond a reasonable doubt."