We live in a world governed not by the biggest creatures, but by the smallest. Our bodies act as vessels for all that we call “ourselves,” forming a barrier between “out there” and “in here.” While that barrier is not as simple as a wall or a single membrane, the philosophy is made real by a complex defense network called the immune system.
As opposed to writing some kind of comprehensive molecular description of the immune system (which I’m sure would bore most of you to tears), I want to share a bit of immunological philosophy, as much of it as we understand anyway. If I leave something out, please don’t crucify me in the comments, just do a bit research on your own (good luck, immuno can be a bit confusing).
The first thing about the immune system that’s good to understand is the duality of “the barrier.” In one sense, you have an inside and an outside. Your skin is considered the absolute first line of defense against your environment. The majority of the things we touch do not get through our skin because it is thick (microscopically speaking), dry, and, in most cases, doesn’t have any openings. Typically you need to have a wound in order for a microbe to breach the skin. Exceptions include some parasites that can essentially “bite” you.
While your extremities and midsection are protected from most organisms by skin, there are several orifices that are easily entered by bacteria and viruses. A good example is your nose or mouth. Lining your respiratory and digestive tracts from “tip to stern,” as it were, are mucosal membranes. These are the front lines against outside attackers. The air we breath, food we eat, and water we drink are chock full of microbes. Many of these invaders simply cannot survive within us and die. Others are quickly stopped by defensive chemicals found in our saliva and other secreted fluids (tears, for instance, can lyse cell walls). Those that make it past these barriers must either invade directly through the mucosal membrane (often colonizing an area near where they entered, like strep throat or thrush) or suffer the riggors of the digestive system.
Here is where the physical metaphor of a barrier breaks down and one must substitute a barrier of vigilance. Any foreign body that makes its way this far will have successfully evaded major components of the “innate” immune response. That is, the parts of the immune system that are ALWAYS ON. The body has a much more specific system for dealing with particularly persistent intruders called the “acquired” immune response – let’s delve into that now. This particular component of the immune system is the part that plagues so many biology students, so I’ll keep to the concepts.
The Two Deadly Flags
Your body has two primary methods of identifying a specific invader. First, any cell, once infected, can gather bits of its attacker and broadcast them to other cells via its outer membrane. A professor of mine once told us to think of this as a “suicide flag,” inviting other cells to come and destroy it. Cells in your body have no stake in their own survival: if instructed to die, they will try to comply.
The second method is a different kind of flag. Only a very select number of cells carry this marker because it is the “sniper flag.” If an immune cell recognizes a foreign body, it has the power to “call in a hit” on that invader, effectively mobilizing itself and others to rapidly divide and form an impromptu army of cells. Even after the invader has been neutralized, these cells remain in the body, to fend off attack in the future (the basis for what most people think of as immunity).
The two flags, really called “MHC I” and “MHC II,” are quite remarkable because they utilize the unique ability of the immune system to recognize almost any organism on Earth. Each relevant immune cell can recognize only a single biological subunit (essentially part of a protein, lipid, or nucleic acid called an antigen) and become activated and ready to rid the body of its target. The immune cells make up for their single-mindedness in numbers. For instance, there are approximately 1,000,000,000 unique B cells in each person.
This process costs the body a lot of energy and stress, though, so the acquired immune response only occurs once the antigen has been confirmed to exist in the body. Some diseases randomly activate immune cells and illicit a very dangerous reaction called a cytokine storm. The deadly Spanish Flu of 1918 and the modern H1N1 flu are known for this behavior.
What does this all mean to YOU?
So what’s the take-away message? Note, I didn’t mention macrophages, complement, antibodies, or even lymph nodes. Why? Because the strength of our immune systems is also the reason they’re so hard to understand: complexity. We have a vast number of organs, cells, and immune particles dedicated to protecting our insides from the bad things in the world. Personally, it makes me glad that our bodies have what it takes to compete in that great big microbial world out there!
Confused? Been swept into thinking more about immuno? Just wondering why antibodies look like big “Y”s? Well please post your questions in the comments or @me on Twitter (contact info at the top of the article)!