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determines which elements are tolerated, and therefore can remain part of that
living thing, and which elements are rejected, and therefore cannot remain part
of that living thing. But what exactly does the immune system detect in this
process of internalfiltering? Although it has long been said that the immune
system controls the identity of the elements with which it interacts (that is, their
conformity with the self), my own view is that the immune system eliminates
elements that change too abruptly, while tolerating elements that change slowly
(Pradeu et al. 2013). Regardless of the criterion one adopts, though, it remains
that the immune system constitutes such afilter over internal elements in
addition tofiltering the entry of external elements at bodily interfaces.
Lastly, the immune system plays an important role in thepromotion of
cooperationbetween the components of the organism. It does so in two different
ways. First, the immune system can ensure long-distance communication
between remote components of the organism. In plants this is done through
the vasculature and thanks to metabolites (Shah and Zeier 2013). In metazoans
this is made possible by different processes, most prominently the unique
mobility of immune cells. For example, immune cells can inform distant organs
about the presence of a pathogen in the organism and also contribute to tissue
remodeling in remote sites (Eom and Parichy 2017). As we will see inSection 5,
other systems like the endocrine and the nervous systems are also capable of
long-distance communication in metazoans, but the immune system is unique in
its capacity to send cells to every compartment of the organism. Second, the
immune system can eliminate noncooperative elements (“cheaters”), for
instance, cancer cells (see next section) (Michod 1999; Frank 2007), via the

Filtering over entry

Filtering over presence

Promotion of cooperation

Immune system Biological individuality

Countability

Delineation

Cohesion

Functional integration

Near-completedecomposability

Copperation and absence ofconflict

Figure 3.1 The three main activities by which the immune system
participates in the individuation of biological entities and how they map
onto the traditional conception of a biological individual.Filtering over
entry sheds light on the idea of the individual as a countable and relatively well-
delineated entity, whilefiltering over presence and promotion of cooperation
shed light on the idea of the individual as a cohesive entity.

22 Elements in the Philosophy of Biology