either directly or indirectly (through its interactions with the CNS, PNS, endo-
crine system, and/or other systems). The immune system has an impact on
sickness behavior (Konsman et al. 2002) and major depressive disorder
(Dantzer 2018), and strategies are currently being developed to treat several
behavioral disorders by targeting the immune system. Another major example is
pain, a highly discussed topic in the philosophy of the neurosciences
(Hardcastle 1997) and in general philosophy as well. Activated immune cells
release pro-inflammatory cytokines, which sensitize sensory nerve endings,
leading to an amplification and prolongation of pain; but the pain response
also is downregulated via opioid-containing immune cells, as these cells release
opioid peptides, which interact with opioid receptors on sensory nerves (Stein
et al. 1990). All this corroborates the claim that pain is a highly complex process
with several feedback loops and involving several bodily systems (Hardcastle
1997). Last but not least, many researchers propose that the immune system, via
microglia, T cells, and cytokines, participates in cognition, particularly in
learning and spatial memory.
All these data contribute to question brain-centered and more generally
nervous system–centered views of behavior and cognition. Clearly, the nervous
system remains crucial in all the processes described above, but it would none-
theless be inadequate to consider only the nervous system when trying to
identify the biological basis of feelings, emotions, behaviors, and cognitive
states. Understanding these processes requires an integrative approach in which
the immune system could play an important role. Accordingly, neuroimmunol-
ogy lends more weight to the idea of embodied cognition, that is, the idea that
bodily elements distinct from the brain play a significant role in cognitive
processing (Shapiro 2010). It also offers additional arguments to those who
emphasize the importance of interoception (Craig 2002) insofar as it suggests
that the brain can form a representation of the immunological status of periph-
eral tissues.
One may wonder, though, whether the immune system is an essential com-
ponent of behavior and cognition. Here the answer will depend on what exactly
is meant by this question. If the question is whether the immune system could
influence behavior and cognition on its own, that is, independently of the
(central and peripheral) nervous system, then the answer is probably negative.
But if the question is whether the immune system is essential for proper
functioning of the nervous system in some behavioral and cognitive activities,
then the answer is affirmative, as exemplarily illustrated by microglia-mediated
synaptic remodeling. More specifically, I suggest that the immune system can
play three kinds of specific roles that are essential for the functioning of the
nervous system and particularly for the realization of some cognitive processes.
62 Elements in the Philosophy of Biology