Immunology as a Metaphor for Computational ... - Napier University

Chapter 3. Immune Systems for Scheduling 563.5.4 Evolution of the Gene LibrariesA genetic algorithm is used to evolve a set of immune libraries as shown in figure3.2 in exactly the same manner as [Hightower et al., 1995]. Each individual in thepopulation represents a complete set of libraries, i.e. an entire immune system. As in[Hightower et al., 1995], a haploid representation is used in which the total number ofgenes in each individual is l c s¥ . Each AIS in the initial population is generatedby assigning a random value to each gene. The fitness of an individual is determined£by its overall ability to produce schedules which optimise T max across all the potentialscenarios described in the antigen universe, i.e. against all potential antigen encounters.The procedure by which fitness is calculated is a modified version of that given in[Hightower et al., 1995], as described in chapter 2, section 2.2.2.1. A set of antibodies(schedules) are expressed from an individual by combining one component from eachlibrary (see section 3.5.2 of this chapter) and then exposed to the antigen universe.For each antibody-antigen encounter, a schedule is constructed, and the quality of theschedule in terms of T max measured. Each antigen receives an antigen-score whichis the minimum, i.e. the best, of all the values of T max measured for that antigen.The overall fitness of an individual is computed by averaging all the antigen-scores;this assumes the survival probability of an individual depends on all the pathogenicchallenges it encounters. An alternative approach would be to take the view that in factthe survival probability is dependent on the extent to which it is able to deal with themost difficult encounters, and therefore its fitness is characterized by the worst valueof T max found during an antigen encounter. The exact algorithm for computing fitnessis given in figure 3.3.After a match-score for an antibody has been calculated, the antibody is mutated atrandom in M positions, and the match-score recalculated. If the match-score improves,then the original antibody is assigned this new match-score. The mutations are notwritten back to the gene library.