5.4 Mapping the Different Conceptual Questions Raised
by Neuroimmunology
Recent scientific literature abounds in bold claims about the relation between
the nervous and the immune system. These two systems are often said to have
strong interactions and many similarities, perhaps a common evolutionary
origin, while some suggest that they overlap to a great extent and even that
they could constitute a single system (see details below). I suggest that, even if
several of such claims are often formulated as if they were intimately connected
or even equivalent, it is crucial, for the sake of clarity, to disentangle these
claims. My analysis of this literature has led me to single outfive dimensions of
neuroimmunology, each corresponding to a different question (Figure 5.4):
interaction, similarity, overlap, origins, and control. Importantly, distinguishing
five questions in neuroimmunology can serve not only as a mapping of the
various goals currently pursued by researchers in this domain but also as an
invitation to do further investigations to better address some questions that have
tended to remain in the background in the last two decades.
5.4.1 Interaction: How Do the Nervous and the Immune
System Interact?The question of how exactly the nervous and the immune system interact is the
most basic and the most extensively discussed in neuroimmunology. Some of
the main results of this research have been presented in the previous two
sections. My aim here is not to give more details about this question but rather
to explain why it should not be confused with the four other questions that are
often raised in the scientific literature.
5.4.2 Similarity: Are the Nervous and the Immune System
Structurally and/or Functionally Similar?The nervous and the immune systems are often said to be similar, that is, to
share several important features. Similarity can be at a structural level. Both
systems communicate via soluble ligands and receptors. Molecules that
mediate such communication include cytokines, chemokines, neuropeptides,
neurotransmitters, neurotrophins (Camacho-Arroyo et al. 2009), and their
receptors. Perhaps more distinctively, both systems make use of specific
structures called synapses. The term appeared in neurobiology in the late
nineteenth century and was subsequently adopted by immunologists in the
1980s to describe the extended communication surface platforms established
between two immune cells, particularly antigen-presenting cells and lympho-
cytes (Steinman 2004). Both types of synapses are stable adhesive junctions
54 Elements in the Philosophy of Biology