Welcome to first chapter of A Lymph’s Life! Since this is a story of cells, during the first chapters we will explain how and where our immune cells are born: The Hematopoiesis!
What’s the Hematopoiesis?
Hematopoiesis is the process by which blood cells (erythrocytes, platelets and immune cells) form and develop. They are all generated by a common precursor cell, known as a hematopoietic stem cell.
In the majority of cases cell division gives rise to a pair of identical daughter cells, but in some instances cell division is asymmetric, i.e., two different types of cells result from the process. This is the case when stem cells divide. One of the resulting daughter cells retains the characteristics of the original cell, while the other differentiates — acquiring novel characteristics and losing the capacity to generate other cell types.
If we take a look at the family tree above, we can see that five different types of cell arise from just one cell type, and for this reason these precursors are known as pluripotent hematopoietic stem cells.
Although each of these five cell types is also a stem cell which maintains its ability to divide asymmetrically, they give rise to a smaller variety of cell types than pluripotent stem cells, i.e., they have lost pluripotency. We’ll take a look at them below:
these give rise to erythrocytes (red blood cells) which are in charge of transporting oxygen in the bloodstream. Their job is essential to the survival of all other types of cells in the body. Their shape is a bit like that of a donut — but without the hole — and characteristically lack a nucleus.
give rise to thrombocytes (or platelets) which are in charge of shoring up holes in the blood vessels which would otherwise cause hemorrhaging. They are also capable of generating signals that attract cells of the immune system and which allow the repair of damaged tissue. They are very small, irregularly shaped and, just like erythrocytes, lack a nucleus.
Also known as myeloblasts. These cells give rise to granulocytes, which are a type of immune cell. Granulocytes have various functions and participate in the struggle against bacterial and fungal infections, the elimination of parasites, and the production of warning signals. They are also responsible for the bothersome symptoms of allergies. There are three types: eosinophils, basophils and neutrophils — which we will talk about later on.
Generate lymphocytes and NK cells, both of which are also immune cells. There are two types of lymphocyte — T- and B-lymphocytes — and these are responsible for the specific immune response. Despite arising from the same precursors, the maturation process for each of these cell types is completely different. NK cells are similar to T-lymphocytes, but form a part of the nonspecific immune response.
These cells give rise to monocytes/macrophages, which are other components of the immune system. Monocytes are generated in the bone marrow and afterwards travel through the bloodstream to various tissues. In case of danger they leave the bloodstream they rapidly turn into macrophages, and their job is to eliminate microorganisms or the leftovers of infected or cancerous cells.
As you can see, each one of the cell types generated in the bone marrow plays an important role which requires close cooperation between it and all the other cells types for the correct functioning of the immune system.
Now take a look at the small lymphocyte in the last box of the episode above.
“Apoptosis — take me!” he says.
What is apoptosis? We will find out very soon, but for the moment, remember that word, because it is a process that is very important for the body’s continued survival . . .