In this and the next few episodes, we’re going to take it very slowly, given that we’re learning about some processes in which many components of the immune system participate and many names are thrown up — it can be difficult to take it all in.
As we have seen, from one pluripotent hematopoietic stem cell (the little old grandma in the rocking chair we saw in the last episode) multiple immune system cell type precursors can be generated in the bone marrow. Each one of these cell types undergoes a maturation process which we will see unfolding step by step later on. It goes without saying that all these different processes have steps in common.
At the initiation of these processes, the signals that the precursors receive from the environment in which they find themselves are very important: This environment is composed of the cells of the organ in question — usually called stromal cells. These signals are molecules which promote precursor growth and division, and kick off the processes of differentiation and maturation. Some of the signals are released by cells, others are molecules which are found stuck to cells, and require contact with the cells around them in order to receive these signals. These signals are called cytokines.
What is a Cytokine?
Some cytokines assist in the maturation or activation of cells of the immune system, others serve as an alarm signal when the body is under threat…
Many of these molecules are known, as indeed are the cells they affect. For example, in the third cartoon above we see that one of our precursors is under the influence of something called GM-CSF, and the other is receiving IL-7 signals. Knowing only this we can, in the manner of Sherlock Holmes, deduce what these young cells are destined to become as grown-ups.
The cytokine GM-CSF (Granulocyte/Macrophage Colony-Stimulating Factor) we see the cell munching on above tells us that the cell will grow up to be either a granulocyte or a monocyte, although for the moment we can’t tell which.
IL-7 (interleukin-7) is another well-known cytokine. In the bone marrow IL-7 stimulates the first step on the journey of B-lymphocyte maturation. Therefore, we can deduce that our young friend above will in time become a brave artilleryman.
All of this looks pretty complicated, but don’t worry because further on well take a more detailed look at what a cytokine and other similar molecules are — and we’ll go through the most important ones.
To finish up, let’s take a look at the last cartoon. It is important to know that not all cells undergo the same type of maturation:
- Some go through long processes involving many steps and a lot of examinations along the way, for example B- and T-lymphocytes.
- Others, from the moment they appear, resemble very much the type of cell they will become in the future, and so their maturation is more straightforward. This is because T- and B-lymphocytes are highly specialized regarding the type of enemy they target (they must learn to recognize their enemies well and tell them apart from friendly elements), while non-specific cells learn their roles more rapidly, as happens with our precursor who appears in the last cartoon: from a very young age he feels a certain compulsion to eliminate whatever he doesn’t recognize as self…
After reading what this last little cell says you will be asking yourselves what an MHC is. Once more the message is that you will learn all about MHCs in great detail further on. But the simple explanation is that all our cells have what are called MHCs (or Major Histocompatibility Complexes) which are something like unique identity cards which prove that a cell is a “citizen” of the body.
There are immune system cells capable of recognizing other cells which do not possess this ID — NK (Natural Killer) cells. As their name would suggest, NK cells are specialists in eliminating any cell that they regard as potentially problematic. And so the third precursor which appears in today’s episode is none other than a young NK cell.
In the next episode we will discover that not all the precursor cells of the immune system go through these processes in the bone marrow; some must travel through the body until they reach the far-flung tissues where they will be able to mature and grow into valiant soldiers.