Information Technology Reference
In-Depth Information
ports are also provided. For the purposes of discussion we will assume that
monitored networks are using the familiar TCP/IP protocol suite. Consequently
we shall refer to these functions as “src”, “dst”, “srcport” and “dstport.”
2.2
Danger Theory
Over the last decade the focus of research in immunology has shifted from the
adaptive to innate immune system, and the cells of innate immunity has moved
to the fore in understanding the behaviour of immune system as a whole[2].
Insights gained from this research are beginning to be appreciated and modelled
at various levels by researchers building artificial immune systems.
The algorithm described in Section 3 incorporates at a conceptual level mech-
anisms from both the innate and adaptive immune system although, because of
the change in problem domain, these are implemented differently. This section
briefly reviews the biological processes and mechanisms which have been drawn
upon when designing the algorithm presented in this paper.
The biological immune system as a whole provides effective host defense
through the complex interaction of various immune system cells with themselves
and their environment, the tissue of the host organism. Dendritic cells (DCs),
part of the innate immune system, interact with antigen derived from the host
tissue and control the state of adaptive immune system cells.
Antigen is ingested from the extracellular milieu by DCs in their immature
state and then processed internally. During processing, antigen is segmented and
attached to major histocompatibility complex (MHC) molecules. This MHC-
antigen complex is then presented under certain conditions on the surface of the
DC. As well as extracting antigen from their surroundings, DCs also have recep-
tors which respond to a range of other signalling molecules in their milieu. Cer-
tain molecules, such a lipopolysaccaride, collectively termed pathogen-associated
molecular proteins (PAMPs[3]) are common to entire classes of pathogens and
bind with toll-like receptors (TLRs) on the surface of DCs.
Other groups of molecules, termed danger signals, such as heat shock proteins
(HSPs), are associated with damage to host tissue or unregulated, necrotic cell
death and bind with receptors on DCs. Other classes of molecules related to
inflammation and regulated, apoptotic cell death also interact with receptor
families present on the surface of DCs. The current maturation state of the DC
is determined through the combination of these complex signalling networks.
DCs themselves secrete cell-to-cell signalling molecules called cytokines which
control the state of other cell types. The number and strength of DC cytokine
output depends on its current maturation state.
T-cells, members of the adaptive immune system, have receptors which bind to
antigen presented in an MHC-antigen complex on the surface of DCs and respond
to the strength of the match between receptor and antigen. This response is
usually a change in the differentiation state of the T-cell. However, this response
is not solely determined by antigen, but also by the levels of cytokines sensed by
a range of cytokine receptors present on the surface of T-cells. These receptors
are specific for cytokines produced by DCs.