The body protects itself in many ways. Some involve the immune system but other mechanisms act at the boundaries of the body, dealing with microbes before they enter. The eyes protect themselves with blinking and antiseptic “tears” for instance. Mucus in the nose traps and washes away microbes and other particles. Saliva is slightly acidic, making the mouth inhospitable to germs. There are many more examples. A recent experiment has identified a previously unknown method that operates in the upper airways. It has is a rather unexpected link to the sense of taste.
I have been aware for a few years now, that some people experience bitter tastes much more readily than I do. While I am happily chomping through a mild-tasting portion of broccoli, the person on the other side of the table is screwing up his face in disgust, and forcing it down. Apparently there is a gene that makes some people much more sensitive to the taste of bitter compounds than others. This might have a slight evolutionary advantage because bitterness can be associated with poisonous vegetation and fungi. So ancestors with this gene might have been less likely to die young from accidental poisoning. But it seems that it might bring an additional advantage when it comes to a bacterium that can cause pneumonia (Pseudomonas aeruginosa).
This airborne bacterium can stimulate bitterness receptors not on the tongue, but in the upper airways. In a laboratory experiment, tissue taken from human sinuses not only sensed the bitter taste of the bacteria but also produced nitric oxide to kill them. The author suggested to New Scientist recently that people with a greater ability to detect bitterness might as a consequence, have a better defence against this particular bug.
Those of us who are not so sensitive to bitterness can console ourselves that it is easier for us to eat our greens and so gain some health benefits from that source.