Nano in the Environment: Allergens

Spring is here! We’re very excited for the weather to warm up and for our flowers to bloom (we’ve got several planters of tulips and hyacinths on their way!). We’ve also got a gardenia that we love and are trying to help survive in Wisconsin. Folly? Probably.

What we’re not so excited for, however, are the seasonal allergies. With a stuffy nose and itchy eyes, I struck out to find out if there’s any nanoscience behind allergens. It turns out there is a lot of nanoscale stuff involved! To get a better understanding of what nanoscale means, take a look at this blog post imagining shrinking a human down to the nanoscale!

Porch photo with several potted plants
Shameless photo of our plants. Some tulips are beginning to bloom in the foreground, the gardenia is struggling in the background, and the hyacinths aren’t yet visible in the railing baskets! (photo by Curtis Green)

When you first think of allergens, what comes to mind? For me, I think of pollen, animal dander, and dust. Just so happens that I love plants and my dog, and every home has dust, so we’ve got a nice trifecta of allergens to keep our sinuses going. When I looked into these allergens though, I found out a couple of surprising things. First, house dust is a lot of dead skin cells and dust mite poop.1 EW. Time to get some more air purifiers.

Two black and white microscope images. Left: (a) spiky round blob labeled P. salicifolium with 10 micron scale bar. Right: (b) blob with rough surface and knobby shapes on the end, 20 micron scale bar
Check out these scanning electron microscope (SEM) images of what some allergens actually look like: a) pollen and b) dust (image a from Huang et al., 20173 courtesy of open access; image b from Wilson, 20144 courtesy of Open Government License)

Going more in depth, I was curious about the size of allergens, and where nano-stuff might be involved. Lo and behold, I was not the first person to ask this question, and I found this neat chart summarizing particulate sizes! 2 For most allergens, the particulates appear to be micrometers large, at the very edge of where human eyes can see things. That may seem small, but remember, nano is way smaller than that! As a reference point, human hair is tens to hundreds of micrometers in width. Particles that find use in sunscreens, cosmetics, or electronics can be more than 100,000x smaller than that!

chart showing size on the x-axis (0.0001 to 1000 micrometers) and type of particle on the y-axis (in order of increasing size: gas molecules, particulate contaminants, types of dust, and biological contaminants)
A chart comparing the sizes of different particulates in micrometers. 1000 micrometers = 1 millimeter (image by Jisaac9)

I found another surprise during my research, too: I had always assumed that allergens affected our airways and elicited an allergic response because of their size. It turns out the answer is more complicated than that! While the size of airborne particles is important in determining if they irritate your airways or cause other health effects,5 the real culprits in allergic responses are proteins.6 Not the kind of proteins you find in your protein shake, though (I would not recommend trying to bulk up with pollen, that sounds miserable).

image of strawberry smoothie, with dandelion photo superimposed with arrow pointing into smoothie
A smoothie modified with some dandelions for protein. I don’t recommend trying this. (Smoothie image by Marco Verch; dandelion by Steven Pavlov)

Proteins are simply a type of large molecule with specific shapes and functions (more on that in a second). The proteins that cause allergic responses are different from the ones we eat in that they are the targets of antibody responses in humans. With vaccines being everywhere in the news, it’s likely you’ve heard of antibodies, which are our bodies’ way of destroying things that might harm us. Particles like pollen, dust, and dander are all just vehicles for these problematic proteins, which are much smaller than the particles!

This is where the nanoscale comes into play with respect to allergens. You can think of proteins as a long string of little building blocks called amino acids. The string then ties itself into knots (protein folding) so that the long string turns into a nano-sized clump. To get an idea of what that looks like, take a look at the image below showing a common protein found on dust called Der p 1.6,7 Just how small are these folded proteins? Take a look at the dust particle above. It is close to 50 micrometers across, so it is probably too small to see with the naked eye. The size of Der p 1 when it is folded is around 0.7 nanometers,7 which means you could stack over 71,000 Der p 1 proteins back to back across the width of that microscopic dust particle! With that kind of size difference, it is easy to see how particles like dust and pollen can pack a lot of allergy-causing proteins!

colorful protein ribbon diagram
A cartoon showing the structure of Der p 17 after it has tied itself into a crazy knot. Can you trace the line from beginning to end?? (Image from RCSB protein databank )

              With proteins as the main allergens, how can we avoid them? They are too small to be filtered out by our typical air filters… The good thing is that proteins don’t just fly around in the air by themselves: they need larger particles like dust and pollen to carry them around,6 and we can filter dust and pollen! That is the reason why air filters are effective at reducing allergens: they trap the particles that carry allergy-causing proteins. This is just how N95 masks and HEPA filters protect us from COVID-19! They trap particulates (like tiny water droplets from our breath) that are on the order of a micron in size, 8 so we don’t get exposed to the viruses in the water droplets. So, the next time you notice your allergies flaring up, you’ll know to blame those pesky proteins!


References

  1. Wikipedia. Dust. 2022. https://en.wikipedia.org/wiki/Dust
  2. Engineering ToolBox. Particle Size: The size of dust particles, pollen, bacteria, virus, and more. 2005. http://www.engineeringtoolbox.com/particle-sizes-d_934.html
  3. Huang, Y. et al. Taxonomic status and phylogenetic position of Phaeostigma in the subtribe Artemisiinae (Asteraceae). Journal of Systematics and Evolution 55 (5), 2017, 426-436. doi: 10.1111/jse.12257
  4. Wilson, H. Seeing the invisible – striking images of dust! 2014. https://blog.nationalarchives.gov.uk/seeing-invisible-striking-images-dust/
  5. USEPA. Health and Environmental Effects of Particulate Matter (PM). https://www.epa.gov/pm-pollution/health-and-environmental-effects-particulate-matter-pm
  6. Woodfolk, J.A. et al. Allergens, sources, particles, and molecules: Why do we make IgE responses? Allergology International 64 (4), 2015, 295-303. doi: 10.1016/j.alit.2015.06.001
  7. Meno, K. et al. The crystal structure of recombinant proDer p 1, a major house dust mite proteolytic allergen. Journal of Immunology, 175, 2005, 3835-3845. doi: 10.4049/jimmunol.175.6.3835
  8. UBC. What size particle is important to transmission of COVID-19?https://www.aerosol.mech.ubc.ca/what-size-particle-is-important-to-transmission/