Updated: Apr 2
2020's Star Body System
We’ve been hearing a lot of talk about the immune system in the past year or so. In fact, I think it’s safe to say that no other body system has received as much media attention, possibly ever. One could argue that “cytokines” was one of the top words of 2020, rising from relative obscurity to become a household world. A layperson with some general scientific knowledge
might be able to identify white blood cells as key players in the immune system, and someone who’s a good trivia team member might be able to recognize specifically that T cells are often responsible for eliminating some of the body’s most dangerous invaders. But how do T cells know which pathogens to target?
The Hidden Life of Cells
Before I talk more about the workings of the immune system, I’d like to expand a bit on what I wrote in my previous blog post. I listed a few things that cells do which make them similar to living organisms such as humans, but that list was by no means exhaustive.
One thing we do that cells also do is communicate with each other. Cytokines for example a type of chemical messenger that allow cells to influence other cells over relatively long distances; there are also hormones, neurotransmitters, and many others.
But perhaps more interestingly, cells can communicate by directly handing material to other cells. Sounds strange? Am I saying that cells have little arms they use to hold things and which can extend out to the receiving arms of another cell? Well, kind of…
In a human civilization, there are a variety of important jobs that we need people to do. There are also professions among the trillions of cells that form a human body. Which brings me to the main subject of this post, dendritic cells — critical but under-appreciated cells often described as “professional antigen-presenters.”
To give some perspective on how little attention these cells typically receive, the textbook I used to teach anatomy devotes exactly two sentences to dendritic cells in its 700+ pages. As we’ll soon see, this is unfortunate as these cells exhibit several fascinating behaviors, but aside from being really interesting (and looking cool), they also play a major role in keeping us healthy.
You don’t need to look too deeply to find dendritic cells. In fact one place where they’re commonly found is just below the surface of the skin in the lower layers of the epidermis. The epidermis serves to keep invaders out of the body and is often the site where immune responses are initiated. The cells of the epidermis are tightly packed together to provide protection and waterproofing, but dendritic cells are able to crawl their way through these cells by squeezing and propelling their skeletons forward (yes, cells have skeletons too).
Wherever they’re found, the principal action of dendritic cells begins with sampling the body’s environment for the presence of antigens – invaders that result in the production of immune antibodies. They do so through a variety of processes including phagocytosis and pinocytosis which literally mean “cell eating” and “cell drinking”. Pinocytosis involves taking little sips of the fluid which surrounds cells, and a cell does this by extending its membrane around some of this fluid and bringing it into the cell. If you haven’t seen a video of pinocytosis you’re definitely missing out — here’s a link with some incredible footage of this process.
Arming the Body's Defensive Weapons
In another example of how the life of a cell mirrors that of an organism, dendritic cells go through a maturation process which begins when they stop sampling for antigens and begin to migrate towards sites in the body designated for carrying out immune response, lymph nodes and the spleen. When dendritic cells enter these sites, cells which carry out attacks against invaders such as T cells are also considered to be immature or naive, meaning that they don’t have instructions regarding which pathogens to attack. That all changes during the fascinating process of antigen presentation.
The dendritic cells have carried little fragments of pathogens into the immune tissues and these fragments are held on protein arms which extend out from the surface of the cells. Dendritic cells then encounter immune cells where a series of what we can think of as handshakes takes place. The naïve immune cell receives the pathogen fragment and other reactions take place which facilitate this exchange.
During this exchange, the dendritic cell tells the immune cell what it needs to know to carry out the immune response – what type of invader the fragment came from and where in the body it can be found. Armed with this knowledge, the activated immune cell can now travel to the site of the infection and begin attacking the invader. One dendritic cell can activate thousands of immune cells in this manner thus allowing the immune system to mount devastating attacks against the invaders that cause sickness.
New Directions for Dendritic Cells
It makes sense that medical practitioners want to harness and amplify the ability of dendritic cells to activate immune cells, and for this reason treatments involving dendritic cells can be used in cancer therapy. One technique involves harvesting dendritic cells from a patient and growing them in laboratory conditions in such a way that trains them to recruit other immune cells to fight tumors. Clinical trials are underway to use this technique in the fight against several types of cancer, and dendritic cell therapy can be used on its own or in combination with other approaches like chemotherapy.
While the medical establishment recognizes the potential for dendritic cells to make an important positive impact on our health, these cells have yet to receive general recognition for their important contributions and for their role in allowing the better known immune cells to carry out their functions. Dendritic cells give us insight into some of the ways in which the life cycle of cells is similar to that of an organism and also show some of the amazing things cells are able to do. As we continue to direct our attention to the immune system, perhaps these crucial but under-appreciated cells can gain the recognition they deserve in anatomy textbooks and beyond.