Two flavors of immune cells: myeloid and lymphoid

The distinction between myeloid cells and lymphoid cells

We recently covered the fact that hematopoietic (blood) stem cells can make all the blood cell types in the body, including white blood (immune) cells. The immune cells are broadly broken down into two categories: myeloid and lymphoid.

 

Myeloid cells

Myeloid cells include neutrophils, macrophages, dendritic cells, mast cells, and granulocytes. These cells tend to be found in the tissues- your skin, organs, and other locations where an infection might rear its head. They have three main jobs.

1. Pick up information from their environment, which will tell them if any threats are nearby.
2. Kill infected cells and pathogens like bacteria.
3. Communicate with other immune cells, including lymphoid cells, sounding the alert when danger is detected.

Myeloid cells are part of the innate immune system.

Lymphoid cells

Lymphoid cells, or lymphocytes, include T cells, B cells, and natural killer (NK) cells. As the name suggests, these cells are prominent in the lymph, a fluid that drains from the tissues containing cells and debris. These cells tend to have a large nucleus and few granules (the pockets that contain their ammunition). Lymphocytes mount a more sophisticated, targeted attack against threats than myeloid cells. B cells and T cells are part of the adaptive immune system, while NK cells are known to have properties of innate and adaptive immune cells.

We think about myeloid and lymphoid cells separately, since they are made and live in distinct areas in the body, plus they have different jobs. But as we continue, you’ll see how closely linked they must be to keep us healthy- no immune cell is an island.

Next time we’ll talk about the key differences between the innate and adaptive immune responses. As you may guess from the names, innate immunity uses techniques that cells are “born” with to fight infections. Adaptive immunity requires adaptation. For me, that’s where things start to get really interesting!

Drop me a line in the comments if you have thoughts about which type of cells, myeloid or lymphoid, would respond first if you cut your finger.

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!

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