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

Chapter 2. Background 122.1.1 The Network HypothesisStudies have shown that each antibody has a specific antigen determinant known asthe idiotope — this gives rise to a possibility first articulated by Jerne in [Jerne, 1973]that antibodies can recognise other antibodies as well as antigens, resulting in a large,self-regulating and mutually reinforcing network of antibodies.This is shown in figure 2.2. In this diagram, the idiotope of B-Cell1, ID1 stimulatesB-Cell2, and the two become connected via the paratope of B-Cell2, P2. Thus,ID1 is acting as an antigen from the viewpoint of B-Cell2, and this causes B-Cell2to suppress the antibodies produced by B-Cell1. On the other hand, ID3 acts as anantigen from the viewpoint of B-Cell1, and is recognised by BCell1s paratope, P1, andthus ID3 stimulates B-Cell1 to produce antibodies. Hence, a large chain of suppressionand stimulation can be set up between B-Cells, resulting in a self-organising andself-regulatory network. Importantly, the network is not fixed, but varies continuouslyaccording to the dynamical changes in the environment. This is known as the metadynamicsof the system [Varela and Coutinho, 1988], and is realised by incorporatingnewly generated cells into the network and removing useless ones. The new cells aregenerated when cells in the existing network are stimulated and proliferate, resulting insome mutant species, and also owing to gene-recombination in the bone marrow. Despitethese dynamic perturbations, an underlying core network of B-Cells is thoughtto be maintained by the immune system, representative of the antigens to which it hasbeen exposed. The network remains stable due to the suppression mechanisms whichprevent the over-stimulation of B-Cells.2.1.2 Clonal SelectionThe clonal selection theory considers that each lymphocyte, whether B-Cell or T-Cell,is capable of recognising essentially one kind of antigen. When an infectious agentis encountered, a few of the many circulating B-Cells recognise it. Those cells arethen induced to proliferate rapidly until within a few days there are sufficient numberof them to mount an adequate response. This process by which an antigen selectsfor and generates the specific clones of its own antigen-binding cells is known