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presented to lymphocyte receptors by APCs. h erefore, such APCs act as fi lters
that can extract the important information and remove the molecular noise.
Learning and memory . A major feature of the adaptive immune system is that
it is able to learn through its interaction with the environment. h e fi rst time
an antigen is detected, a primary response is induced, which includes prolif-
eration and subsequent reduction of lymphocytes. Some of these lymphocytes
are kept as memory cells. h e next time the same antigen is detected, memory
cells generate a faster and more intense response (secondary response). Accord-
ingly, memory cells work as an associative (highly) distributed memory.
Diversity . Clonal selection and hypermutation mechanisms are constantly
testing diff erent detector confi guration for known and unknown antigens.
h is is a highly combinatorial process that explores the space of possible con-
fi gurations looking for close-to-optimum receptors that can cope with all
types of antigens. Exploration is balanced with exploitation by favoring the
reproduction of promising individuals.
Distributed processing . Unlike the nervous system, the immune system is not
centrally controlled. Detection and response can be executed locally and
immediately without communicating with any central organ. h is distrib-
uted behavior is accomplished by billions of immune molecules and cells that
circulate around the blood and lymph systems and are capable of making
decisions in a local collaborative environment.
Self-regulation . Depending on the severity of the attack, response of the
immune system can range from very light and almost imperceptible to very
strong. A stronger response uses a lot of resources to help ward off the attacker.
Once the invader is eliminated, the immune system regulates itself to stop the
delivery of new resources and release the used ones.
Self-protection . By protecting the body as a whole, the immune system is also
protecting itself. It means that there is no other additional system to pro-
tect the immune system; hence, it can be said that the immune system is
self-defended.
2.7 Summary
h is chapter discusses some theoretical models based on abstract immunological
processes. Among these processes, the clonal selection theory has been studied since
the 1950s to understand how certain types of B and T lymphocytes are selected for
destruction of specifi c antigens invading our body.
h e IN theory has been investigated extensively by many theoretical immu-
nologists, and three generations of this theory were developed to simulate complex
dynamic behavior of the immune system. Another approach used multiepitope IN
model and mapped into a PDP as a neural network. h e GC is a complex system and
hence, it is di cult to model all its internal mechanisms. h e more the functions of
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