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Accounting for environmental decision making:

Chapter 7

Natural responses to uncertainty


7. NATURAL RESPONSES TO UNCERTAINTY
HOW THE IMMUNE SYSTEM GENERATES DIVERSITY OF RESPONSE

7. Natural responses to uncertainty

"He that will not apply new remedies must expect new evils: for time is the greatest innovator." Francis Bacon, "Essays: 24. Of innovation"

This section will look at how some biological systems handle uncertainty. It will look at the advantages and drawbacks of these approaches and the types of uncertainty they deal with. Drawing analogies from these systems to human systems of decision making requires care. The types of information and energy follows {flows} and the controls needed to maintain systems are especially important to recognise.

Diversity in evolution can in some ways be though of as a method of producing a diverse response to an uncertain external environment . Diversity of response is a diversity of action to the same, or an indistinguishable, stimulus. This diversity comes in many different forms:


How the immune system generates diversity of response

Take for example the immune system, this is a way of the body responding to invasion by pathogenic organisms. Potentially pathogenic micro-organisms typically have generation times many orders of magnitude smaller than their human hosts. Although individual cells in the human body replicate quite rapidly, they contain essentially the same DNA. New combinations of genes only arise with reproduction and the production of a new human. The human body represents a vast source of potential food for pathogens. Because pathogens can reproduce much faster they can evolve to beat the cell's defences.

Except for one thing, the immune system. Cells in the immune system (Lymphocytes) do not all contain the same DNA. Through different methods of generation, they express enough diversity to limit the damage cause by many pathogens. This response is not always perfect, but then there are still humans around (and other organisms with immune systems) so it may be good enough.

Golub and Green (1991) describe the methods with which the immune generates a diverse response. Lymphocytes recognise pathogens with antigen receptors. The receptors contain 'heavy' and 'light' protein chains, assembled from three and two gene segments respectively. The heavy chains contain Variable (V), Diverse (D), and Joining (J) segments, the light chains contain just V and J. There are at least six methods in which these genes are used to express diversity: (Taken from Golub and Green (1991) except where noted)

1. Genetic (allelic) diversity - different V,D and J genes in different humans, (Mattila et. al. 1995)
2. Genetic diversity - large numbers of V,D and J genes per organism
3. Recombinatorial - random choice of V(D)J segments expressed in each lymphocyte
4. Junctional - imprecise joining of segments
5. Combinatorial - different V(D)J segments in the heavy and light chain of single lymphocyte.
6. Somatic mutation - lymphocyte replication produces mutation in the V(D)J genes (Brezinschek et. al. 1995).

(Also see: Generation of diversity in the immune system)

Is this a method for matching internal or external uncertainty? In some ways it is both. The system has to recognise foreign cells, and the generation of mutation in those cells is not something in can affect (external uncertainty). But, the foreign cells evolve in the environment of the body and the ones that survive the immune response are the ones that reproduce (internal uncertainty).

There are limits in taking this further, these cells are all working within a single body, if the body dies without reproducing, they cannot reproduce for themselves. Human systems contain individuals that can reproduce with the aid of only one other individual (well some of them still remember how to do it without hospitals). There are further limitations when we consider that certain viruses can subvert this system and cause autoimmune diseases (Adams 1996). The immune system cannot destroy all cells, it must also recognise the bodies own cells. This presents a limitation of the response it can make. The time taken to detect a foreign cell also limits its effectiveness.


[Chapter 8 - Modelling responses to uncertainty]

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Finished 13/9/96
Created 18/9/98
Modified 20/6/99