39 percent of 40- to 70-year-old women, and thyroid tumors in 36 to 100 percent
of 50- to 70-year-old adults (Welch and Black 2010).
So, if tumors are so frequent, why do only a limited number of them turn into
actual cancers? The reason we don’t get more cancer is that several systems of
regulation exist in the organism, systems that prevent the formation of
a cancerous tumor, its growth, and its invasion of distant tissues (metastases)
(Klein et al. 2007). These regulation mechanisms are diverse and located at
different molecular, cellular, and tissue levels (Hanahan and Weinberg 2011):
they include DNA maintenance and repair (Hoeijmakers 2001), regulation of
cell cycle (Sherr 1996), apoptotic signals sent to cancer cells by surrounding
cells (Letai 2017), and inhibitors of angiogenesis (formation of new vessels)
(Jain 2005), among many others.
The aim of the present section is to show that, among these regulation
mechanisms, the immune system plays a critical role (Binnewies et al. 2018)
and that, conversely, the immune system is certainly involved in every cancer.
The crucial point here is that cancer is a disease of multicellularity–or, more
precisely, cancer is a disease that results from a dysfunction of the mechanisms
that normally insure the cohesion of the multicellular individual, the very
mechanisms among which we have seen that the immune system is crucial
(see Section 3). A striking result of research done in the last twenty years is that
the immune system can bothpreventandsupportcancer formation (de Visser
et al. 2006; Binnewies et al. 2018). Understanding how the immune system,
which we characterized in previous sections as essential for maintenance of the
cohesion of the organism, can in some circumstances promote cancer constitu-
tes a major challenge from a conceptual as well as a therapeutic point of view.
The outline of the section is as follows. First, I explain the progressive
construction of the idea that the immune system can restrain cancer. Second,
I explore how the immune system was subsequently described as capable of
both restraining and promoting cancer. Third, I sum up current views about
immune–cancer interactions and their clinical applications, especially immu-
notherapies. Finally, I examine the role of the immune system in the breakdown
of biological individuality often described as typical of cancer.
4.1 How the Immune System Restrains Cancer: The Complex
History of the Idea of Immunosurveillance
It is obviously difficult to make broad generalizations when discussing cancer.
Most of the time, when we talk about cancer, we refer to a set of significantly
heterogeneous diseases characterized by uncontrolled cell multiplication and
the cells’potential to invade other parts of the organism. The heterogeneity of
30 Elements in the Philosophy of Biology