Biomedical Engineering Reference
In-Depth Information
immunological studies will provide a framework of ideas as to how 'danger' is
assessed in the HIS. The danger signals should show up after limited attack to
minimize damage and therefore have to be quickly and automatically measurable.
Once the danger signal has been transmitted, the artificial immune systems (AIS)
should react to those artificial antigens that are 'near' the emitter of the danger
signal. A number of advantages are claimed for this theory; not least that it provides
a method of 'grounding' the immune response. The theory is not complete, and
there are some doubts about how much it actually changes behavior and or struc-
ture. Nevertheless, the theory contains enough potentially interesting ideas to make
it worth assessing its relevance to AIS. Few other AIS practitioners are aware of the
Danger Theory, notable exceptions being Burgess [
4
] and Willamson [
5
]. Hence,
this deals directly with the Danger Theory. In the next section, we provide an
overview of the Danger Theory, pointing out, where appropriate, some analogies in
current AIS models. We then discuss about anomaly detection for AIS.
Danger Theory
The AIS are computational systems designed on the principles of natural Immune
System (IS), which is highly distributed, adaptive and diverse system [
6
]. Danger
Theory is presented with particular emphasis on analogies in the Artificial Immune
Systems world [
7
,
8
]. The idea is that the artificial cells release signals describing
their status, e.g., safe signals and danger signals. The immune system is commonly
thought to work at three levels: External barriers (skin, mucus), innate immunity,
and the acquired or adaptive immune system. As part of the third and most complex
level, B Lymphocytes secrete specific antibodies that recognize and react to stimuli.
It is this pattern matching between antibodies and antigens that lie at the heart of
most Artificial Immune System implementations. Another type of cell, the T (killer)
lymphocyte, is also important in different types of immune reactions. Although not
usually present in AIS models, the behavior of this cell is implicated in the Danger
model and so it is included here. From the AIS practitioner's point of view, the T
killer cells match stimuli in much the same way as antibodies do. It is fundamental
that only the 'correct' cells are matched as otherwise this could lead to a self-
destructive autoimmune reaction. Classical immunology [
9
] stipulates that an
immune response is triggered when the body encounters something non-self or
foreign. It is not yet fully understood how this self-non-self discrimination is
achieved, but many immunologists believe that the difference between them is
learnt early in life. In particular it is thought that the maturation process plays an
important role to achieve self-tolerance by eliminating those T and B cells that react
to self. In addition, a 'confirmation' signal is required; that is, for either B cell or T
(killer) cell activation, a T (helper) lymphocyte must also be activated. Matzinger's
Danger Theory debates this point of view (for a good introduction, see Matzinger
[
1
]). Technical overviews can be found in Matzinger [
2
] and Matzinger [
3
]. Danger
Theory clearly has many facets and intricacies, and we have touched on only a few.
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