Hypermutation
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Hypermutation is the central aspect to making the Acquired immune system possible. Large creatures such as vertebrates typically have a long generation time, while (micro-)parasites (such as viruses, bacteria, fungi or worms) that they play host to often have a short generation time.
This means that the parasites will evolve much faster than their hosts, potentially overwhelming them if there's no rapid defence.
To be able to deal with these attackers, some form of adaptation mechanism is needed.
Organisms with an acquired immune system will pick up invasions using the less efficient innate immune system , and small amounts of dead invader material will be presented to ... cells in the lymph nodes.
These cells unlock a certain part of their DNA related to antibody (alpha-?)unit production, and proceed to rapidly divide, all the while mutating this section of DNA. While dividing, the lymph-cells will auto-select for antibody binding affinity to the invader fragments.
This process takes 3 weeks, and speeds up that which would otherwise require centuries of evolution.
The survivors of this selection process will continue to divide, and go on to produce T-cells, ...-cells, ... , and antibodies, which will now be capable of detecting and destroying the invader with exceptional efficiency.
Also a small number will be distributed to lymph nodes throughout the body as memory-cells. If the same invader ever invades again, the memory cells will rapidly divide, and confer the same defensive capability to the body again. The organism is now immune.
Some worm species avoid the acquired immune response by modifying their external skin or secretions every couple of weeks.
The hypermutation process also has cells auto-select against the organism's own cells: their 'signature'. Failure to auto-select against the own signature causes an auto-immune response, which is often dangerous to the organism itself.
see also:
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