Table 1.

Circulating and Tissue-Localized Immune Cell Types and Their Role in Immunity

		Immune Cell Type			Role in Immunitya
Blood	Innate	Neutrophils			•Phagocytic granulocytes that release soluble antimicrobials during infection
		Eosinophils & Basophils			•Circulating granulocytes involved in allergy/asthma and parasitic infections
		Monocytes			•Circulating phagocytes; involved in antigen presentation and wound healing •Precursors of inflammatory monocyte-derived dendritic cells (DCs) and macrophages
		DCs	Plasmacytoid DC		•Small subset of DCs with lymphocyte-like morphology; critical in anti-viral immunity
		Natural killer (NK) cells			•Cytotoxic for infected and tumor cells
	Adaptive	Circulating T lymphocytes	Naive T cell		•Protect against new infections; primary response
			TCM		•Central memory T cell; circulate between blood and lymphoid tissues; predominantly CD4+
			TEM		•Effector memory T cell; migrate through tissue sites
			TEMRA		•Terminally differentiated effector T cell; predominantly CD8+
			Treg		•Regulatory T cell; immune regulation and homeostasis
		Circulating B lymphocytes	Naive B cell		•Protect against new infections; primary response
			Plasmablast		•Short-lived antibody-secreting B cell generated early in an infection
			Memory B cell		•Patrol for previously encountered antigens; quick and robust secondary response
Tissue	Innate	Mast cells			•Express high-affinity IgE receptors (FcϵRI) that recognize antigens bound to IgE antibodies •Release histamine and other molecules associated with anaphylaxis
		Tissue macrophages			•Professional phagocytes that reside in tissues to ingest cellular debris and foreign material •Highly specialized and distinct depending on their tissue of residence
		DCs	Conventional DC		•Present antigens to induce T cell responses
		Innate lymphoid cells (ILCs)	NK cell		•Anti-tumor and anti-viral tissue immunity
			Helper-type ILC		•Cytokine production profiles match corresponding helper T cell subset •Promote tissue repair and display functional plasticity
	Adaptive	Tissue-resident T lymphocytes	αβ T cell	TRM	•Tissue-resident memory T cell; in situ protection against pathogens and tumors
				Treg	•Suppression of tissue inflammation and tissue repair
				Tfh	•Follicular helper T cell; support B cell maturation, and required for germinal center formation
			Innate-like T cell	γδ T cell	•Express γ/δ T cell receptors
				iNKT cell	•Invariant NK T cell; recognize lipid antigens presented on CD1d molecules
				MAIT cell	•Mucosal-associated invariant T cell; recognize vitamin B metabolites on MR1 molecules
		Tissue-resident B lymphocytes	Plasma cell		•Long-lived antibody-secreting B cell generated later in an infection •Localize predominantly in bone marrow
			Memory B cell		•Long-lived; recognize known antigens for faster, more robust secondary response
			Follicular B cell		•Produce high-affinity antibodies; primarily localize in follicles of secondary lymphoid organs
			Marginal zone B cell		•First line of defense against blood-borne pathogens entering from circulation

^aFor references, see text.