across the animal kingdom. For example, recent work has shown that IL-17,
a pro-inflammatory cytokine, is a neuromodulator and contributes to behavior in
Caenorhabditis elegans(Chen et al. 2017). Thus, much more research is needed
about the history of the nervous and immune systems in all their diversity before
making a claim about their possible common origins.
Second, one cannot make direct inferences from similarity to common
evolutionary origins. There are obviously many different possible evolutionary
explanations for similarities, including homology (shared ancestry) and analogy
(convergent evolution). In future research deciding between these two options
at the system level will be important but it will also be difficult. The question has
also been raised at the cell level: did neural and immune cells evolve indepen-
dently but later co-opt functions from each other, or did they evolve from
a common ancestral cell able to recognize and interpret the environment,
communicate with other cells, and exhibit plasticity (Kioussis and Pachnis
2009)?
Additionally, evolutionary explanations will differ depending on whether we
want to explain interactions, similarities, or overlap between the nervous and
the immune system. This is more confirmation that one should be careful when
switching from one question to the other.
So, exciting research remains to be done for scientists and philosophers
interested in when and why nervous and immune (sub)systems emerged in
evolution and how theyfit together. An interesting proposal has been made on
this topic by my colleague Jean-François Moreau (personal discussions). In his
view, the functioning and origins of the nervous and the immune system must be
put into the context of the emergence of multicellularity. Multicellularity pre-
supposes internal communication and in metazoans three types of long-distance
communication channels can be distinguished. Thefirst resembles our electric
networks; it corresponds to the nervous system, and more specifically to
neurons, which can send information at a very high speed with a relatively
diverse content. The second resembles our water and/or sewage networks; it
corresponds to the blood and lymphatic vessels, where endocrine signals, in
particular, circulate. It delivers information at a relatively high speed with
a relatively diverse content. Thesefirst two systems (nervous and vascular)
are rigid: they can be modified (via neurogenesis and angiogenesis, for exam-
ple) but only at an extremely slow rate. The third system resembles our mail
carriers; it corresponds to immune cells, which are the uniquely mobile cells of
the organism and which can deliver information with extremely diverse content
everywhere in the organism, often over long distances though at a limited speed.
In addition to carrying information, immune cells can perform all sorts of
activities, including pathogen clearance, tissue remodeling, and tissue repair,
Philosophy of Immunology 59