Immunological memory 2

Published on October 29, 2020   18 min

Other Talks in the Series: The Immune System - Key Concepts and Questions

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My name is David Gray. I work at the University of Edinburgh in the School of Biological Sciences. I'm going to talk to you today about immunological memory. In the second part of my talk, I will consider how memory is maintained over long periods of time.
Using these diagrams, which you'll be familiar with, we can think about, what's the best way to keep memory for long periods of time? Remember that memory is the basis of vaccination, and the whole point of vaccination is that you would be protected from reinfection or at least the symptoms of suppresses viral disease second time you're exposed to it, and that, if it's going to be successful, needs to be protection over many years, and that's eminently possible using some of the best vaccines available. They provide protection over 50-60 years, that's readily been observed. Again, looking at these diagrams, there seemed to be a number of different ways in which you could do that. You could maintain the frequency for long periods of time, or you could simply maintain cells that have enhanced function. As we talked about, it's likely to be, and is, a mixture of both the quantity and the quality, the quantity of those memory cells and the quality of those memory cells that's important.
Some introductory remarks about cell survival, one needs to ask the question whether a memory cell is an inherently long-lived cell. In the immune system, are there inherently long-lived cells? Or do memory cells require external stimuli for their long-term survival?
In answering that question, one has to think about lymphocyte lifespan in general, and some factors which control that. For instance, the size of the lymphoid system is controlled within very strict limits. You see an acute expansion of lymph nodes following infection or following immunization, but that returns to normal within a few days. So there is a strong homeostatic control of lymphocyte numbers. Now that's important obviously, because in the immune system, we've constantly got input of new lymphocytes from our primary lymphoid organs, from the bone marrow in our early life, from the thymus as well for T cells. In addition to that, as we've been talking about, there is clonal expansion following immunization, so antigen-driven expansion of populations of lymphocytes. If we'd maintained all of the cells that are generated during those clonal expansion and the aura generated from our primary lymphoid organs, the immune system will just get bigger and bigger and bigger, and come to dominate our body. In fact, we have a strong homeostatic regulation imposed upon the immune system. What that means is that all lymphocytes are under constant selective pressure.