which a higher-level individual prevents proliferation of lower-level individuals
(Godfrey-Smith 2009, pp. 100–103). From that point of view, cancer cells
appear as a result of a“re-darwinization”at the cell level.
However, the mechanistic details by which the multicellular organism exerts
control over cancer cells have remained vague. Michod (Michod 1999, p. 119)
cites programmed cell death and the immune system as the two main“policing
mechanisms”in the multicellular organism, but he does not give any detailed
explanation about how they work. An important lesson of what has been said in
this section is that immunological surveillance constitutes a convincing exam-
ple of a mechanistically precise process of maintenance of cohesiveness in the
organism (Prendergast 2012; Pradeu 2013). Thus, it offers an important con-
tribution to this long-standing debate concerning de-darwinization in cancer.
The details of how immune-mediated control works are well documented: in the
elimination phase of immunoediting, myriad immune cells and molecules
(macrophages, dendritic cells, NK cells,γδT cells, CD4 and CD8 T cells,
IFN-γ, among many others) contribute to the destruction of the tumor (Dunn
et al. 2002). In addition, immune-mediated control contributes to coordinate
other control instruments, such as apoptosis and angiogenesis.
This, however, strengthens rather than solves the paradox: if immune-
mediated restriction is one of the main mechanisms ensuring the cohesion of
the organism, we need an explanation for why in some circumstances the
immune systemfavorstumor development (through the escape phase of immu-
noediting and/or through repair mechanisms that contribute to creating
a favorable tissue environment for the tumor). To better understand this phe-
nomenon, I propose here an extended view of immune-mediated cohesion and
decohesion.
The traditional view about immune-mediated cohesion, as represented in
classically defined immunological surveillance, is that the immune system can
eliminate abnormal cells such as cancer cells (Figure 4.2).
This view, however, is too narrow, because it neglects the diversity of
activities in which the immune system is involved, which include not only
defense but also development, tissue repair, clearance of debris, and mainte-
nance of tissue homeostasis, among others (seeSection 2). In cancer, many of
these immunological activities are found. This leads to a much richer view of
immune-mediated cohesion and decohesion in cancer (Figure 4.3). In this view,
the immune system plays a major role in regulation of the organization of the
local tissue, and, together, the immune system and the tissue realize different
activities, including the elimination of abnormal cells, but also thecontainment
of abnormal cells (in that case, cells are not destroyed, they are simply kept
under control, which limits the damage they can do and/or their capacity to
38 Elements in the Philosophy of Biology