I didn’t always want to be a scientist…

But I’ve always been fascinated by the human body. As a kid, I dreamed about the possibility of going inside and traveling where the blood flows, letting myself drift through the bloodstream, visiting different organs along the way.

What I didn’t realize back then was just how many different and highly specialized cells make up our blood.

If I had to guess, many of you probably think of red blood cells when you think of blood. And yes, we have a lot of those! In fact, just one cubic millimeter of blood (that’s a tiny cube of blood) contains around 4 to 6 million red blood cells. It’s hard to even imagine that many cells packed into such a small space. But wait — don’t be too impressed just yet. The best is still to come.

Photo by Photo By: Kaboompics.com

 

More Than Just Oxygen Delivery

Besides its essential job of delivering oxygen to every part of our body, blood also carries immune cells, our personal army of defenders that fight infections and keep us healthy. These immune cells are collectively called white blood cells, and they come in all kinds of shapes and sizes.

Their different appearances actually reflect the different jobs they do. Together, they work as a team to protect us from infections, as well as other threats like cancer.

 

Meet Some of Your Fighters

Neutrophils

These are some of the weirdest, and coolest, cells in your immune system. They’re like your body’s first responders. When an infection happens, neutrophils are among the first to arrive at the scene. One of the wildest ways neutrophils fight bacteria is by literally spitting their guts out! When they detect danger, they can release their own DNA to trap invading bacteria in a sticky net. It’s a dramatic and noble death — but highly effective. Insane, but awesome, right?

Monocytes that then become macrophages

Monocytes are immature immune cells that are present in your blood. Once they leave the blood they can become macrophages. These are your big eaters! “Macrophage” literally means “big eater.” They protect you by devouring bacteria and other harmful invaders through a process called phagocytosis. Think of them as Pac-Man cells — opening wide and gobbling up anything that doesn’t belong.

Lymphocytes

These cells hold a special place in my heart. Even though they look a bit “boring” under the microscope (tiny little round cells), they’re incredibly powerful soldiers.

Lymphocytes include T cells and B cells, which have a huge range of jobs. One of the most famous roles is that B cells produce antibodies, and T cells can help kill infected and even cancerous cells.

There are other important immune cells in our blood, but I’ll save those for future posts.

 

How Do We “Make” Blood?

Here’s something amazing: All the different types of blood cells come from the same kind of cell.

Wait, what?! Stefanie, you just told me these cells look and act so differently. How can they all come from the same cell?

Yup, it’s true. All the cells in our blood — red blood cells, white blood cells, and platelets — come from a special cell called a hematopoietic stem cell. As always, don’t worry about the complicated names. Let’s focus on the concept.

A stem cell is like a baby cell. And just like babies can grow up to become anything — a doctor, a teacher, a barista — stem cells can develop into many different types of cells in the body. But they don’t become a fully formed professional overnight. Like us, they go through stages of development.

These hematopoietic stem cells live inside our bones (yes, inside!) This space is called the bone marrow. There, they get special signals that guide them through the steps of becoming more specialized.

Unlike humans, who can (theoretically) switch careers at any point in life, stem cells become more limited in their options as they develop. Once they commit to a certain “cell fate”, there’s no turning back. They gradually specialize until they reach their final state.

Even though the development of all blood cells starts in the bone marrow, many of them finish maturing in other parts of the body. If we could somehow hop inside our blood vessels, we’d see a beautifully complex mix of cells, our very own microscopic army, working around the clock to keep us safe.

 

Keep Asking Questions and Seeking Answers!

Your immunologist (in training),

Stefanie Valbon, MSc, PhD candidate

 

 

P.S. If you’re part of the population eligible to donate blood, this is your sign to check out nearby blood drives. If you’re in Quebec, Canada, you can find donation info here: Héma-Québec

 

(*) Also important to note: blood not only carries nutrients and oxygen, it also removes waste products and contains platelets — tiny cells that help stop bleeding by forming clots when a blood vessel is damaged.