The birds and the bees of immune cells

Where do immune cells come from?

Before we get into the logistics of how the cells of our immune system do their jobs, let’s talk about where they come from. You may be more familiar with the term white blood cells, which also describes immune cells. White blood cells share a common ancestor with red blood cells and platelets, the hematopoietic stem cell.

Hematopoietic stem cells

The term stem cell has become loaded with political baggage recently, but has a rather simple definition. When a stem cell undergoes cell division, which is to say it splits into two cells, it makes one stem cell (self-renewal) and one cell that is more specialized or developed. Self-renewal and the ability to give rise to all the cells in a family (for instance, all blood cells) are the hallmarks of stem cells. The hematopoietic stem cell (HSC) is able to make all the cells in the hematopoietic (or blood) family. HSCs are typically found in the bone marrow, the inner core of the bones.

An overview of blood cell development

Progenitors

When HSCs divide to give rise to one HSC and one more committed cell, the more committed cell is termed a progenitor. A progenitor is not a stem cell, as it can no longer give rise to all of the blood cells. A progenitor is also not a mature immune cell, ready to do battle with an infection. It is at an intermediate phase, capable of making a variety of closely related cells, but no longer able to make more distantly related cells.

In the bone marrow, common myeloid progenitors give rise to myeloid cells, the early response team of the immune system, as well as red blood cells and platelets. Common lymphoid progenitors make lymphoid cells, including B cells, natural killer cells, and T cells. However, the bone marrow cannot make T cells. Lymphoid progenitors have to leave the bone marrow, travel through the bloodstream, and go to the thymus, an organ that specializes in making T cells.

We’ll build on these concepts and diagrams as we go. All questions welcomed in the comments!

<p><span style=”display:none”>claimtoken-5357cf15993bb</span></p>

What is immunology?

Perhaps you already know that immunology, or the science of the immune system, is a study of the processes that keep you well and help you get better. You catch a cold because you ride the bus to work everyday and someone is always coughing on the bus. (Just, you know, hypothetically. But I’m looking at you, guy in the flat cap.). Your immune system senses the virus, fights it, then braces itself for another bus commute.

Immunology is, indeed, the study of how you stay healthy and fight infections, but that simplicity greatly underestimates the work of the immune system. Have you ever thought about what it entails for your immune system to sense and fight a virus?

The immune system must make sense of a constant stream of signals

• How does your immune system know it’s dealing with something threatening (virus) as opposed to something unfamiliar but harmless (food, air impurities)?
• How do your cells know that they’re dealing specifically with a virus instead of bacteria, fungi, or cancerous cells?
• Which of your immune (or white blood) cells are involved?
• How do your cells (tiny) find the offending pathogen (teeny tiny) in your body (relatively vast)?
• What do your immune cells do once they locate their targets?
• How do they know when to stop?

Immunology is the study of myriad cell types and several organs, which must be coordinated to protect you from a constant onslaught of nasty microbes. The immune system must determine between harmless and harmful stimuli. It must mount an appropriate response against harmful stimuli, by coordinating various cells with specialized functions. It must turn that response off in order to eliminate threats without causing too much collateral damage.

This is only the briefest introduction to what the immune system does and some of the questions that immunologists study. Hopefully it gave you some food for thought about the complexities of immunological self-defense. We’ll start to get into the science in the next post.

Stay healthy out there, bus commuters.