an infection or an autoimmune disease remains crucial. It is reasonable for
future neuroimmunological research, therefore, to study both similarities and
interactions even when they do not go hand-in-hand.
5.4.3 Overlap: To What Extent Do the Nervous and the Immune
System Overlap or Even Constitute a Single System?Sometimes, comparisons between the nervous and the immune system go
beyond mere similarity, especially when functional features considered as
typical of one system are exhibited by the other system. A key discovery was
that immune cells express receptors for neuromodulators and neurons express
immune receptors, which is crucial for their capacity to influence each other.
Cytokines, classically attributed to the immune system, are also produced by
cells of the CNS (microglia, astrocytes, and neurons), where they regulate the
development of the nervous system as well as some of its most crucial physio-
logical processes, including neurotransmission (Camacho-Arroyo et al. 2009;
Ransohoff 2009). Conversely, neuropeptides, long thought to be specific to the
nervous system, are now known to be produced by immune cells as well
(Steinman 2004). Because of this overlap, traditionally conceived functions
are often blurred: elements classically defined as pertaining to the nervous
system play important immunological roles and vice versa.
Neuroimmunologists frequently put together the“interaction”question and
the“overlap”question (Ordovas-Montanes et al. 2015; Chavan et al. 2017).
However, overlap does not always come with interaction, as illustrated by many
cases of functional redundancy (where identical or nearly identical components
can realize a given function) and functional diversity (where heterogeneous
components can realize a given function) in studies on robustness (Kitano
2004b). Two engines in an airplane overlap functionally, but it is preferable
that they don’t have too strong an interaction, otherwise damage to one motor
might disrupt the other. Such fail-safe mechanisms abound in biology.
Reciprocally, interaction does not necessarily imply an overlap. If a cytokine
produced by the immune system interacts with the nervous system, it is impor-
tant to study it, even if it is not produced by the nervous system as well.
Furthermore, scientists enthusiastic about functional convergences between
the nervous and the immune system tend to switch from the idea of overlap to the
idea of unification, that is, the idea that the nervous and the immune system
(together with the endocrine system, in general) constitute one single system
(Petrovsky 2001; Steinman 2012). But here too, caution is in order: clearly two
systems can overlap without constituting a single system. This is illustrated,
again, by studies on biological robustness. Many cases of functional redundancy
Philosophy of Immunology 57