the activities of immune cells include but are not limited to defense, comprising
also development, repair, clearance of debris, and so on (Michell-Robinson
et al. 2015).
The long-dominant view has been that, in healthy conditions, the brain is
devoid of lymphocytes, as these cells could cause major damage. Recently,
however, a “peri-cerebral” adaptive immune system has been described
(Figure 5.2): the meninges contain lymphatic vessels that remove waste from
the parenchyma, can relay information about possible infections in the brain,
but also harbor peripheral immune cells that communicate with the brain via
cytokines (Louveau et al. 2015b;Kipnis 2016). This discovery confirms that
Figure 5.1 Microglia and their many activities.Microglia are a major element
of the brain’s immune system. These resident immune phagocytes constantly
monitor their microenvironment and participate in many processes in health and
disease, including engulfment of neural progenitor cells (NPCs), acute response
to CNS damage, engulfment of synaptic material, and homeostatic surveillance.
(Figure drawn by Wiebke Bretting).
Philosophy of Immunology 47