Blood is a useful substance, not just for our life, but for our way of thinking.

It's served as a convenient metaphor for so many ideas, covering the extremes and even the contradictions of our experiences. It signifies life, but also accompanies death.

It unites those who share it, but in doing so it divides others. It runs hot, it runs cold. Whatever it is we need to describe, blood is there for us to project onto, flowing through us.

And yet under the microscope, blood doesn’t seem that remarkable. The colors may be vivid, but the cells are so simple and similar to one another. How is it that our complicated, contradictory sense of self, recorded in literature and painting and film… how does all that come down to a fluid made up largely of the simplest cell in our body— a cell so singularly focused on its task of carrying hemoglobin that it actively has to get rid of the organelles and DNA inside of it?

But if we track the origin of these cells, we get a more complicated picture of what they mean to us and to our bodies, and even to our own identity. Our blood is a soup made up of cells swimming around in water, salts and proteins. There are platelets, tiny fragments of cells that help clot our blood when a vessel breaks.

Then there are white blood cells, which help protect our body from infection. But the cells that are the most prevalent in our blood are red blood cells. They’re pigmented red by hemoglobin, which helps the cells transport oxygen to our tissues and organs, and then transports carbon dioxide back to the lungs.

The blood you’re seeing now came from James, our master of microscopes, who had just been scratched by his cats. So you can see the red blood cells that came from the wound, along with the white blood cells. And to better see those white blood cells, James added something else to his slide: spit.

That spit carried bacteria, fresh targets for the white blood cells. In our bodies, white blood cells have a very specific job: they decide what belongs there, and what does not. They seek out molecular markers on bacteria and other microbes that have become familiar to our bodies over ages of evolution.

There are different types of white blood cells, and they vary in the types of organisms they look for and how they deal with them. But our favorites might be the white blood cells that, like the one in the middle of our screen here, consume their target. Immunity is often described in war-like terms, but I like to think of it as something else: identity.

In effect, the white blood cells are part of an elaborate mechanism that decides what is our body, and what is not. They shape our identity from within, like a bouncer who decides on the vibe of a nightclub based on who they let pass. But bouncers occasionally make mistakes, and so do white blood cells.

The consequences for white blood cells are usually more dire, especially if they begin to turn on us and attack our own bodies, leading to autoimmune disorders. Their task of deciding what belongs to us and what doesn’t is a precarious one, a task that still requires outside input as we attempt to shape their responses with medication or treatment. In comparison, the tasks of red blood cells are much more straightforward.

They carry oxygen and carbon dioxide throughout our body. But despite their very different lives, the origins of platelets, red blood cells, and white blood cells have a significant overlap. They are all— with the exception of a few white blood cells— made in the bone marrow, where they start as myeloid stem cells.

We describe stem cells as young and immature, but what we really mean is that they have potential— the potential to take on one of a number of identities once they begin maturing. As the cells divide, signals that describe what the body needs shape what they become. If we are low on oxygen, the bone marrow makes red blood cells.

If we're sick, it makes white blood cells. And these identities require distinct processes to make them possible. For example, to make themselves capable of holding more oxygen, red blood cells get rid of their nucleus and other organelles as they mature, expelling them for white blood cells called macrophages to clear up.

Consider that a healthy human adult is producing 2 million red blood cells every second. Now imagine 2 million nuclei being spat out throughout your body. And it falls on our macrophages to clear them out, to ensure that these bits of us that were definitely once us, now no longer are.

That simplification makes the red blood cells so capable at their singular task. But what if that wasn’t their only job? What if they could do more?

Well, they might. Scientists recently reported finding evidence that red blood cells might work alongside white blood cells and the rest of our immune system, using receptors to find signs of infection or tissue damage, and to signal that to the rest of the immune system so it can act. They may even sacrifice themselves to macrophages to trigger the alarm.

This work is relatively new though, and there is still a lot to learn about the role red blood cells play in immunity. And the more we learn about blood, the more malleable our metaphors and the more complicated the interplay between identity and immunity become. But of course, identity is dynamic.

How can it not be, when it flows through our veins? Thank you for coming on this journey with us as we explore the unseen world that surrounds us. The names on the screen are our Patreon patrons, these are the people who have decided to directly support this channel over at patreon.com/journeytomicro, so we always want to take this time at the end of our videos to thank them for their support.

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