Biomedical Engineering Reference
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terize cytokine actions, and one that is sometimes more useful than characteriz-
ing cytokines as inducers of commands.
Insight into the reasons for pleiotropic properties cI-cIV of cytokines is
provided when they are regarded as providers of information. For example, it is
appropriate that the same cytokine-encoded information induces different re-
sponses in different cells, for different cell types should play different roles in
different situations. In one situation cell type should fade away, as it cannot
help, while cell type should multiply and migrate to the site of the threat. In
another situation, the roles of and should be interchanged. (An analogy is a
scenario wherein hearing the chords of a marching band attracts Sunday drivers
with children to entertain and repels those on the way to catch a plane.) For ex-
ample, ligation of the LPS receptor induces the secretion of a panel of cytokines
(see cI); these carry the message "Gram-negative bacteria are present." In re-
sponse, a variety of appropriate actions are induced in different cell types (see
cIII). That a given response can be triggered by a variety of cytokines (cIV) can
be understood since several different sensed conditions, and hence several dif-
ferent cytokine-coded information packets, could well trigger the same cellular
action (e.g., switch from IgM to IgG) as part of an appropriate response.
With respect to their command function, it is easy to find out what cells
produce the various cytokines (see, e.g., appendix II in (11). By contrast, in-
creased understanding of the role played by cytokine-borne information requires
adding to available knowledge on what cytokines are induced by various recep-
tor ligations.
5.
CONTENDING WITH MULTIPLE INDEPENDENT GOALS
In §3.2 and 3.3 it has been implicitly assumed that the two goals—"kill
dangerous pathogens" and "avoid harm to self"—are both aspects of a single
comprehensive goal of avoiding harm to the host, whether by pathogens or by
the immune system. Yet, this assumption may not be ideal, for different types of
harm are effected by pathogens and by the immune system itself.
Perhaps it is better to regard the two goals (which are a theoretical con-
struct) as independent. Assuming multiple independent goals certainly seems a
sensible way to characterize the interconnected physiological system that deals
with respiration, circulation, digestion, metabolism, neurological information
processing, etc. Indeed, just the metabolic subsystem is faced with the multiple
task of keeping myriad important chemicals at appropriate levels. A related
metabolic task is allocating resources between maintenance and growth (12).
We have made progress in constructing a biologically plausible method to
improve performance under the assumption that different performance goals are
completely independent (N. Rappaport and L. Segel, unpublished). The idea is
to adapt the "algorithm" used by bacteria that tend to swim toward certain
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