Switching off, remembering and the second meeting with a pathogen
The immune response must also be switched off. After the antigens have been destroyed, suppressor T cells bind to the plasma cells, the helper T cells and the cytotoxic T cells. They release lymphokines, proteins that stop the response, so the plasma cells stop making antibodies. Many helper and cytotoxic T cells then die, but some are kept in the lymphatic organs, ready for any similar infection. Acquired immunity has two stages. In the primary response the immune system meets a new pathogen. B cells and T cells that fit its antigens must multiply, which takes time: the first response needs 5 to 10 days to reach its peak. During this time the infection can spread, and the symptoms of the disease may appear. The active B cells and T cells of a response live only a few days, but the primary response also produces memory B cells and memory T cells, which live for tens of years and may last for life. If the same pathogen enters again, the secondary response is very fast. Memory cells are far more numerous than the B cells and T cells that answered the first time, so they recognize the pathogen at once, divide quickly into plasma cells and active T cells, and usually destroy the pathogen before any symptoms appear. For some diseases, such as measles, this is why one illness leaves long protection. The same idea of memory is behind vaccination, and which vaccines a person needs is a decision for a doctor or a health clinic.
| Primary response | Secondary response | |
|---|---|---|
| When | first meeting with a pathogen | same pathogen again |
| Cells that answer | few B and T cells that must multiply | many memory cells |
| Speed | slow: 5 to 10 days to the peak | very fast |
| Result | infection can spread; symptoms may appear | pathogen usually destroyed before symptoms |
Draw a response graph, with a grown-up. On squared paper draw a horizontal axis for days and a vertical axis for “strength of response”. Draw the primary response as a curve that rises slowly to a peak after about a week and then falls. Draw the secondary response as a curve that rises much sooner and higher. This graph shows the idea only; it is not real data. Write one sentence under it that explains the difference with memory cells.