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When we first discovered taste receptors, those proteins your taste buds use to spot different flavors, we assumed their sole purpose was to, well, taste things, but we've since found them in places outside of the digestive system like your lungs, and spoiler alert, that's not to inform you that you just literally inhaled your meal.  Instead, they seem to play a key role in protecting you from pathogens.

On the tongue, each taste sensing cell specializes in detecting just one type of taste: sour, sweet, salt, umami, or bitter.  Three of these, sweet, umami, and bitter, are tasted by proteins called taste receptors, which jut out of the cell.  Each is like a lock that's opened by a certain chemical key.  When the right kind of key comes along, the receptor tells the cell to fire off a taste signal to the brain.

Cells that sense sweet or umami have basically one type of receptor which responds to a handful of chemicals with similar shapes, but bitter sensing cells in your mouth are covered in a diverse assortment of bitter taste receptors, collectively referred to as T2Rs, and each can have several chemical keys that unlock them, so there's a bunch of different things that can make you want to spit.

There's also considerable genetic variability within each bitter taste receptor gene or TAS2R.  Slight variations in a gene's sequence can code for slightly different surface receptors and since you get one version of each gene from each parent, you may end up with two different versions for a given TAS2R.  When this happens, you can sense even more bitter chemicals.  All this variability helps explain why some people love black coffee and strong IPAs while others find them too bitter to swallow, and scientists have generally thought that such diversity exists because bitter taste evolved to tell us what's dangerous to eat.  After all, harmful substances come in all shapes and sizes, so it makes sense that our bitter sensing cells would cast a broad net, but it turns out these same receptors are found in lots other places like your respiratory system and that calls into question the idea that they exist solely to help discern harmful foods, though the body may still be using them ot pick up on dangerous things. 

There's mounting evidence that these receptors alert your lungs to unwelcome guests.  For example, sensory cells in the airway express multiple T2Rs.  When they're triggered, the cells release microbe fighting peptides.  Also, ciliated cells, the ones with hairlike projections that help keep mucus moving, have a different set of T2Rs which sense molecules that bacteria use to communicate.  When they are triggered, the cilia beat faster, which helps move pathogens up and out, and weird as it might seem, some of the variations of T2Rs that influence how you taste bitter substances are associated with higher rates of specific respiratory infections like chronic rhinosinusitis, so your sensitivities to bitter food could predict your susceptibility to certain illnesses or even how well different medications might work to treat them, and targeting these receptors might help treat chronic respiratory conditions like COPD or asthma.

In fact, the more we learn about T2Rs, the more it seems like maybe we shouldn't call them taste receptors.  T2Rs and other so-called taste receptors can be found in all sorts of weird places, including organs that make no contact with the outside world, like the brain and testes, and when researchers figure out why these receptors are in those places, they might realize that taste wasn't their first job and it isn't the most important thing that they do.  

Our ability to sense a broad palette of potential toxins could be more like icing on the cake, so to speak.  

If you enjoyed learning about bitter taste receptors, you might like our episode on the most bitter tasting substance we know of, and here's something sweet to end on: a big virtual hug for our Patrons.  You guys are the best and we couldn't make these videos without you, so super duper triple thanks with a cherry on top, and if you want to learn more about joining our Patron community, all the deets are at patreon.com/scishow.  Thanks for watching.

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