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Infection and immunity
Infection and immunity

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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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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.