Behind enemy lines
Your body is undeniably a battlefield. On the open plains of your skin, respiratory tract, gastrointestinal system, and genitourinary system, scouts from opposing armies are constantly probing for weak points. Bacteria, viruses, and other pathogenic (disease-causing) organisms poke and prod at these barrier tissues, attempting to squeeze their way into the warm, nutrient-rich kingdom that is you. For this reason, immunologists typically consider these tissues - your skin and 'mucous membranes' (the type of tissue that composes your respiratory, gastrointestinal, and genitourinary tracts) - to already be a part of your immune system. These cells are your barrier between inside and out, your Great Wall of China.
But every so often, a scout finds a weak spot. And the pathogenic army infiltrates.
If you've been following along with Microbial Mondays for a while, you might be able to guess what happens next. Traditionally, immunologists describe the defences of our bodies to be a 'two-armed' system, including the so-called "innate immunity" and "adaptive immunity". I like to think of innate immunity as our own personal "Distant Early Warning Lines". During the cold war, a system of radar stations in Canada's frigid far north, which would permit advance warning of any Soviet attacks. If incoming bombers were detected, this arctic line of radar would provide sufficient notice to the more populated southern centers to identify the exact threat, prepare their defences, and possibly mount a counter-attack. Indeed, our innate immune system acts as a warning system that permits preparation for a subsequent, more nuanced defense to pathogenic invaders. But it is also more than that: in and of itsself, it also constitutes a line of early, non-specific defense.
When the innate immune system is activated by the infiltration of an enemy scout, it gets moving right away. Innate immune cells like macrophages and dendritic cells will start eating up any of the pathogenic soldiers that they can find - literally gobbling them up. As they eat, these immune cells also have two other tasks. First, they send out signals to additional immune cells that call them to the site of the problem. These other cells will help mount an attack against the enemy, and eventually promote healing of any wounds at the location of this battle. Secondly, they digest their prey. And upon digesting the soldiers that have entered, they pull a rather medieval trick.
Much in the way ancient harbours would publicly gibbet, or hang in chains, the bodies of pirates and other criminals in an attempt to deter future misdeeds, innate immune cells like macrophages and dendritic cells display the digested remains of the pathogenic army. Bits and pieces of the unlucky bacteria, virus, or other pathogen will be studded across innate immune cell surfaces as a warning to 'all ye who dare to enter here'.
But for these clever cells, it's more than just a simple warning. It's also a call to action for the second arm of the immune system: adaptive immunity.
These satiated and scout-studded innate immune cells travel to central hubs in the immune system called lymph nodes. If you've ever felt your tonsils (which are simply lymph nodes in your neck) swell up as you start to notice the onset of a sore throat, it's because your immune cells are rushing in. In my mind, inside every lymph node, scenes from the Star Wars Cantina on Mos Eisley are playing out. These immune organs are where everybody goes to trade information, and to to hire mercenaries. Although they are highly effective in their own way, the scout-studded dendritic cells or macrophages are only intended to be the first line of defense. It is at the lymph nodes that they hire a more targeted team of assassins: the adaptive immune cells.
This system is incredibly elegant. By eating up the invaders as quickly as possible, innate immune cells not only take out some of the enemy, but they also gather information on what they look like. They pass this information on to the adaptive immune by way of their gibbet-like, cell-surface display of digested pathogens. The information is slowly spread, assassin adaptive immune cells are grown and trained, and then set out just like Luke Skywalker to destroy the Death Star... ahem... bacteria. This also explains why it takes around a week for you to start feeling better when you encounter a new pathogen. It simply takes a few days for the assassins to be hired, and to completely clear out the infiltrators. Luckily for us, though, the adaptive immune system also has a memory - which means that these assassins have a memory. If they see signs of a second Death Star in the body, they already know exactly what to do, and will be able to act on shorter notice from the Distant Early Warning Lines.
Until next time - when we'll get into the third arm of the immune system!