The upshot is that the self–nonself theory is inadequate or at least incomplete,
because many self components trigger immune responses and many nonself
components are actively tolerated by the immune system. Therefore, today’s
immunology calls for novel and richer theoretical frameworks (Janeway 1989;
Grossman and Paul 1992; Matzinger 2002; Pradeu et al. 2013), as well as
renewed reflections about immunological individuality. The next section
shows why, despite the critique of the self–nonself theory presented here, the
claim that the immune system is pivotal for the definition of biological indivi-
duality remains entirely valid.
3.3 Immunology’s Contribution to the Definition
of Biological Individuality
At the most general level, the issue of biological individuality consists in
asking what makes up a countable, relatively well-delineated, and cohesive
entity in the living world (Sober 1991; Hull 1992). (Being countable and
well-delineated concerns what can be called external unity, whereas cohe-
sion has to do with internal unity.) Yet the meaning of countability, delinea-
tion, and cohesion remains unclear and is therefore a matter of debate
from that of healthy individuals, is this difference a cause or
a consequence of the disease?).
Not only has immunology been one of the most activefields in the
development of this recent research on symbiosis (e.g.,Hooper and
Gordon 2001), but it also offers a specific perspective on this topic,
namely the question of how so many genetically foreign entities can be
tolerated rather than eliminated by the host immune system (Xu and
Gordon 2003; Pradeu and Carosella 2006b; Round and Mazmanian
2009 ). In most cases, the symbiotic dialogue between the host and
microbes is mediated by immune interactions (Round and Mazmanian
2009 ; Belkaid and Harrison 2017), which confirms the central role of the
immune system as an interface with the environment as well as between
components of the organism. One striking result is that microbes, long
seen as what the immune system must reject, can often participate in the
immunological defense of the host against pathogenic entities (Pamer
2016 ), thus creating a form of“co-immunity”(Chiu et al. 2017). This
phenomenon of microbiota-mediated protection often involves compo-
nents of the virome (all the viruses living in/on a host), which increasingly
appears as a major future frontier in biomedical research (Virgin 2014).20 Elements in the Philosophy of Biology