Table 1.

Types of immune cell discussed in this review.

Peripheral immune cells
Cell type	Location	% of blood leukocytes	Numbers in blood (${10}^3/\mu \mathrm{l}$)	Main immunological role
Neutrophil	Peripheral blood	16* (mice)	1.4* (mice)	Recruited to sites of infection/injury to kill via phagocytosis, release of granular contents or via pathogen trapping in extracellular traps; usually the first peripheral cells recruited to inflamed tissues.
		60** (human)	3.4** (human)
Monocyte	Peripheral blood	5* (mice)	0.4* (mice)	Patrolling phagocytic cells involved in clearance of debris, phagocytosis, capturing and killing microbes once recruited to tissues.
		5** (human)	0.26** (human)
B lymphocyte	Peripheral blood	77* (mice)	Total lymphocyte count : 6.87* (mice)	Differentiate into plasma cells or memory B cells upon antigen recognition to produce antigen-specific antibodies for long-lasting immunity against secondary antigen challenges.
CD4+ T lymphocyte	Peripheral blood and CSF	27** (human)	1.52** (human)	Recognize major histocompatibility complex (MHC) class II on APCs and aid recruitment of other immune cell subsets.
CD8+ T lymphocyte	Peripheral blood and CSF			Recognize MHC class I and kill via secretion of cytokines, cytotoxic granules or by inducing apoptosis.
Major tissue immune cell types
Cell type	Location	% of CNS leukocytes		Physiological role
PVM	Perivascular space, meninges, and choroid plexus	$\sim 10$ ***		Myeloid cells closely associated with the vasculature which scan the perivascular space, phagocytose debris and initiate recruitment of peripheral leukocytes.
Microglia	Brain parenchyma	$\sim 80$ ***		Resident immune cells of the brain that patrol the parenchyma, send out processes toward sites of injury to confine damage; scavenge foreign material, debris and synapses for removal; involved in synaptic pruning, neurogenesis and axonal growth.
DC	Meninges and choroid plexus	$\sim 3$ ***		Role in antigen capture, processing and presentation to T cells to propagate immune responses.
Mast cell	Meninges, choroid plexus, and parenchyma	$\sim 0.5$ ***		Derived from haematopoietic stem cells, long lived and resident cells, where they can interact with glia cells to orchestrate inflammation. Role not fully determined.

^*8–10 week old male mice.²⁸

^**Male adults.²⁹

^***2 month old C57BL6 mice as assessed by mass cytometry.³⁰