Hypermutation

Hypermutation is the central aspect to making the Acquired immune systempossible. Large creatures such as vertebratestypically have a long generation time, while (micro-)parasites(such as viruses, bacteria, fungior 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.

Organismswith 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 B cells in the lymph nodes.

A B cell that recognizes the antigen is stimulated to divide. During this proliferation, the B cell receptorlocus undergoes mutation at an extremely high rate compared to the normal rate of mutation across the genome. The exact mechanisms for inducing this hypermutation are not clearly understood. It is possible that it involves certain DNA repair enzymes. Dividing B cells compete for growth factors. Those with B cell receptorsthat bind to the antigen strongest get the majority of proliferation stimulating cytokines. Thus, superior B cell receptors generated by this hypermutation survive and proliferate, while the negative or neutral mutants die via apoptosis. This develops a set of B cells, specific for the antigen, with B cell receptors (and thus antibodies) with very high affinities for the antigen (affinity maturation).

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 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-immuneresponse, which is often dangerous to the organism itself.

See also

• Immune system
• Evolutionde:somatische Hypermutation

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It uses material from the http://en.wikipedia.org/wiki/Hypermutation Wikipedia article Hypermutation.