and neurotransmitters that organize physiological and pathological behavior.
(Dantzer et al. 2008)If you fall asleep reading this Element, this is probably simply because you are
tired or because its content is not entertaining enough. But if you are literally
falling asleep, then perhaps you have narcolepsy, a disorder estimated to affect
around one in 2,000 people. The symptoms of narcolepsy, which usually begin in
adolescence or early adulthood, include daytime sleepiness and, in some cases,
cataplexy–sudden muscle weakness during wakefulness that causes falls. Severe
forms of narcolepsy are associated with abnormally low numbers of neurons that
produce hypocretin, a protein that controls sleep–wake cycles. Recently, it was
suggested that narcolepsy might be the consequence of an autoimmune response
(following older work on association with some HLA alleles). Narcoleptic
patients have immune CD4+memory T cells that target peptide fragments of
hypocretin, suggesting that autoimmunity could play a role in narcolepsy,
although the exact causal relationships remain to be determined (Liblau 2018).
A problem that affects many more people than narcolepsy is depression. All
readers of this Element certainly know somebody who has depression and/or
have experienced depression themselves. Depressive disorder, a multiform and
multifactorial condition, was estimated to affect 8.5 percent of people in Europe
(Ayuso-Mateos et al. 2001). For a long time, depression has been considered to
be a psychiatric disease, usually treated with serotonin-tweaking drugs like
Prozac. Yet a growing number of researchers look at depression from an
additional perspective based on immunology (Bullmore 2018; Dantzer 2018)
(more about this example below).
Such examples and many others suggest that the nervous system and the
immune system, far from being separated, can interact intimately in almost all
metazoans, including humans. The study of these interactions has given rise to
an interdisciplinary domain, neuroimmunology. My main objective in this
section is very modest: it is to offer a conceptual clarification of the different
issues raised by neuroimmunology, which often remain intertwined and insuffi-
ciently distinguished. To do so, after a short history of neuroimmunology,
I present important results concerning the interactions between the nervous
and the immune systems in health and disease. I then propose to distinguishfive
different conceptual questions when dealing with neuroimmune interactions,
andfinally I mention some important philosophical consequences neuroimmu-
nology can have, particularly about cognition. So, what will be said in this
section is no more than a preliminary conceptual and philosophical exploration
of thefield of neuroimmunology. My hope is simply to convince some readers
to take up the challenge of immersing themselves in this fascinating domain.
44 Elements in the Philosophy of Biology