libtissue - implementing innate immunity

Jamie Twycross, Uwe Aickelin

Abstract— In a previous paper the authors argued the case for incorporating ideas from innate immunity into artificial immune systems (AISs) and presented an outline for a concep- tual framework for such systems. A number of key general properties observed in the biological innate and adaptive immune systems were highlighted, and how such properties might be instantiated in artificial systems was discussed in detail. The next logical step is to take these ideas and build a software system with which AISs with these properties can be implemented and experimentally evaluated. This paper reports on the results of that step - the libtissue system.

I. IN T RO D U CT I O N libtissue is a software system for implementing and evaluating AIS algorithms on real-world monitoring and control problems. AIS algorithms are implemented as multi- agent systems of cells, antigen and signals interacting within tissue compartments. Input data is provided by sensors which monitor a system under surveillance, and cells are actively able to affect the monitored system through response mech- anisms. libtissue provides a general implementational framework within which many different AIS algorithms can be instantiated, rather thanc [1]. libtissue is being used at the University of Nottingham to explore the application of a range of novel immune-inspired algorithms to problems in intrusion detection. A brief review of the biological and conceptual views that underpin the design of libtissue is given in Section II, more detailed background information can be found in a previous paper [2]. This is then followed by a detailed description of the libtissue implementation in Section III. libtissue has grown into a fairly complex software system and its use is better understood in the context of examples. Thus, Section IV shows how libtissue can be applied to a real-world problem in computer security, and Section V describes the implementation of a simple example algorithm using libtissue. An analysis and evaluation of this algo- rithm are then presented in Section VI. The paper concludes with a brief summary and discussion of future work in Section VII.

II. AP P LY I N G I N NAT E I M M U N I T Y In a previous paper [2] the authors describe several biolog- ical processes in detail and then discuss these biological pro- cesses at a conceptual level. This biological and conceptual view of the immune system forms the foundation upon which the libtissue implementation is built, and a brief summary is given here. The reader is referred to [2] and [3] for further discussions and explanations of the biological terminology.

Jamie Twycross, jpt@cs.nott.ac.uk (corresponding author), and Uwe Aickelin, uxa@cs.nott.ac.uk, are at the University of Nottingham, U.K. The biological immune system is a complex system of cells of different types interacting with each other and the tissue in which they reside. The key elements of the system are cells, signals and antigen, combined with the environ- ment, tissue. Cells have access to their environment through antigen and signals. Essentially, signals provide cells with information on the behaviour of entities in their environment, while antigen provides cells with information on the structure of these entities. In the biological system structure reflected at an antigenic level and behaviour at a signal level are tightly coupled. If the behaviour of a cell changes then so does its antigen profile and vice versa. Part of the motivation for the research presented here comes from a desire to better understand how information from these two levels determines the dynamics of the immune system.

As well as providing information on behaviour, signals also provide a control mechanism for immune system cells. The behaviour of a single cell is determined by complex signalling networks which are actively maintained between cells. A cell’s behaviour can be seen in terms of the functions a cell performs. Of particular interest are the functions of antigen processing, signal processing, cellular binding, antigen matching and antigen response. Simple antigen pro- cessing consists of two steps: antigen ingestion and antigen presentation. During ingestion, antigen is transfered from the extracellular space to the interior of the cell. During presentation, internalised antigen is displayed on the surface of the cell. Additional manipulation of the antigen whilst inside the cell is also possible. A specialised class of cells called APCs performs antigen processing in the body. Signal processing refers to the ability of a cell to have its behaviour influenced through the level of a signal, such as a cytokine or hormone in the extracellular space. Control of DCs by PAMPs and Danger Signals, or of T helper cells by DCs pro

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