Infection and immunity
Infection and immunity

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Infection and immunity

4.5  Adaptive immunity

Adaptive immunity is due to the actions of two types of specialised leukocytes, known as T cells and B cells. (If you are interested, the letters denote ‘thymus’ and ‘bone marrow’, the tissues where each of these leukocytes mature.) We will describe their individual contributions to the adaptive immune response shortly, but first we focus on the most striking difference between innate and adaptive immunity. The clue lies in the word ‘adaptive’.

T cells and B cells have recognition methods that distinguish between different pathogens (e.g. different species of bacteria), and they adapt during their first encounter with a particular pathogen. The second time they meet it in the body, the adaptive response begins earlier, lasts longer and is more effective than it was on the first occasion. You can learn more about this by watching the following animation. (If you do not wish to see closed captions, use the 'CC' (captions) button to remove or reveal the subtitles.)

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INSTRUCTOR: The immune system is said to be adaptive because, after the first encounter with a pathogen, it can develop a much faster response to repeat infection with the same pathogen. This adaptive response is important for vaccination and immunisation. Let's take a closer look at some graphs that illustrate this phenomenon.

This graph shows the first encounter with a pathogen, which might be, for example, the chicken-pox virus. If we chart the number of antibodies and leukocytes the body produces on the vertical axis over time on the horizontal axis, we can see that, after infection at time 0, it takes 10 days for antibody and leukocyte numbers to start increasing. This increase in production of antibodies and leukocytes lasts for just over 15 days.

Now let's take a look at the secondary adaptive immune response by plotting this on the same chart. This occurs with a repeat infection by the same pathogen. In our example, this would be a repeat contraction of the chicken-pox virus. In this case, after infection at time 0 it takes less than 5 days for antibody and leukocyte numbers to start increasing.

The production of antibodies and leukocytes lasts for over 30 days. And it is a noticeably larger increase, compared to the increase seen in our primary response. How is this possible?

The cells of our immune system that are responsible for this phenomenon are known as "B cells." B cells are designed to recognise only a specific pathogen, and so we have billions of them in our bodies. During a primary encounter with a pathogen, the B cell binds to the pathogen via receptors and eventually becomes activated. At this point, it starts dividing, producing copies of itself.

Some of these new cells become "plasma cells." This is the name given to the cells that function as antibody factories, producing antibodies that recognise the pathogen and flag it for destruction by other cells of the immune system. However, these plasma cells only live for a few days.

In contrast, some of the clone cells become memory cells, with a life span of decades. They circulate in the bloodstream, ready to produce antibodies much more quickly when they next encounter the same pathogen. It's the memory cells that produce the secondary immune response.

Thanks to scientific experimentation, we now know that it's possible to deliberately administer a pathogen to generate an immunological memory by the production of memory B cells. It's this process that underlies vaccination and other forms of immunisation.

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So, there is a much faster and increased response to a subsequent encounter with a pathogen and this demonstrates the adaptability of the immune system. This response is due to the production of long-lived memory cells that circulate in the body after the primary adaptive immune response subsides. These memory cells are specifically programmed to recognise the same pathogens that triggered the primary response if they ever get into the body again. You will learn much more about these later in this session.

Overall then it is to be expected that one of the appropriate immune system responses to infection is an increase the concentration of leukocytes in the blood circulation. This expected response can actually be tested in a laboratory by taking a blood sample from an individual who is suspected to be suffering from an infection. Blood from the sample is then smeared onto a microscope slide and air dried, and the sample can then be viewed at different magnifications using a light microscope to enable the number of leukocytes to be counted. In the activity in the next section you can test for the presence of the suspected infection by counting leukocytes in blood samples using our digital microscope.


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