between two cells across which information is transmitted via secretory
molecules. The detailed resemblances and differences between the neural
and immunological synapses are an important object of study (Dustin and
Colman 2002).
The similarity can also be at the functional level. It is often said, for example,
that both systems recognize their target and display a form of memory insofar as
they respond differently to a second stimulation by a given stimulus, as if they
could remember a past experience (Kioussis and Pachnis 2009). Sometimes,
this functional comparison goes very far, as when it is suggested that the
immune system is, in itself,“cognitive”(Tauber 1997). The exact meaning of
this claim is unclear. It dates back to at least Niels Jerne (Jerne 1985). More
recently Irun Cohen (Cohen 2000, pp. 181–189) defended the view that the
immune system perceives signals and responds to them by a decision-making
process, itself based on a complex language and a capacity to remember
(immunological memory). It is uncertain how comparisons made at such
a level of generality may prove scientifically useful because they are difficult
to operationalize and test. They could perhaps generate novel avenues for
research, but they may also be experimentally sterile, as happened to a large
extent with Jerne’s views.
Other functional similarities often mentioned include plasticity and motility
(Kioussis and Pachnis 2009). Both neural and immune cells are plastic to a high
degree: depending on microenvironmental cues, they can differentiate into
phenotypically distinct subtypes. Moreover, cells of both systems are able to
carry information from and to distant parts of the body. Yet as we will see, they
use different means to do so.
Overall, similarities between the nervous and immune systems are numerous
and often informative. Moreover, there are many promising avenues for future
research in this area, including, for instance, a comparison between the pro-
cesses of epigenetic regulation in neural and immune cells.
Neuroimmunologists often switch from the idea that the nervous and the
immune system interact to the idea that the two systems are similar, or the other
way around (e.g.,Steinman 2012; Ordovas-Montanes et al. 2015). It should be
clear, though, that interaction and similarity are two different things. Two
entities can have strong interactions without being similar, and it is perfectly
possible that two similar things have no interaction at all. As self-evident as
such distinctions might seem, they do point to some confusions that exist in the
scientific literature. For example, even if the immunological and the neural
synapses do not interact, it remains extremely interesting to study their simila-
rities. And even if an immune cell at the periphery and a neuron in the brain are
structurally and functionally dissimilar, understanding how they interact during
56 Elements in the Philosophy of Biology