Biology Reference
In-Depth Information
immune system. Of course, we spontaneously tend to think that
these two systems have nothing in common, but if we think of them
in a more abstract way the processes that take place are the same
in both cases. A chemical substance, the nutrient or the antigen,
permits the selective proliferation of an entity, the animal or the
lymphocyte, and in both cases, the substance is broken down dur-
ing interaction with the entity. Usually, the antigen is considered as
a signal to the lymphocyte to proliferate and the nutrient just as
the animal's food. This arbitrary way of creating classes, separat-
ing these chemical substances, depends on our subjectivity. It would
be equally justified to consider the antigen as the lymphocyte's food
and the nutrient as a signal to the animal to proliferate. In reality,
neither of these stances is valid. The nutrition metaphor to describe
these biochemical substances is no more relevant than the informa-
tion metaphor. We must avoid falling into the trap of oversimplifi-
cation. As biology is not a mathematical science, we have become
trapped in the language. For a formal model representing these
processes towards which scientific practice must move, it is neces-
sary to create a new concept to describe what Atamas calls the
'signal-food' (1996, 2003).
To give an objective definition to the signal, the argument is
often put forward that it exerts its effects at very low concentration
and that it has no nutritive value in itself. However, all biochemical
networks are interconnected in a cell. When a cell receives a signal
(or a set of signals) which induces its differentiation and prolifera-
tion during embryogenesis, the chain of reactions participating
in transduction of this signal must end in activating the cell's
metabolism thus permitting the biosynthesis required. Leaving our
subjective classifications aside, what happens here is a series of
chemical reactions interacting one with another, and, in this
respect, what we call signal transduction is nothing but extreme
sophistication of the metabolism which, in the Darwinian model,
permits the cell to adapt to its microenvironment. The signals are
part of it in the same way that trophic substances are. We might
then also call them 'selectors'. In the rest of this topic, we shall,
from habit, call them either signals or selectors.
