Table 1.

Properties and functions of immune cells

Immune cells	Characteristics	% of WBC	Functions	Life span	References
Innate immune cells
Macrophages	Plastic physiology which changes in response to environmental signals Found in all tissues Have proliferative capacity Exist as subsets named according to stimuli inducing their polarization and the cytokine profile they deliver: M1, M2, tumor associated, CD169+, T cell receptor positive	2–8%	1. Clearance of cell debris in tissue remodeling and wound healing 2. Phagocytosis of MCOs and dead or dying cells during resolution of infection or inflammation 3. Antigen-presenting cells (APCs) 4. In response to IFNγ, produce pro-inflammatory cytokines, superoxide anions, oxygen and nitrogen radicals	Several months and years	[33–35]
Neutrophils	Granulocytes Rapidly localize to sites of infection Undergo phagocytosis-induced cell death (PICD) Express CD11b, CD16 and CD66b	40–70%	1. First line of defence against exogenous MCOs 2. Phagocytosis and resolution of infection by PICD 3. APCs	5 h–7 days	[36–38]
Dendritic cells (DC)	Numerous membrane processes that protrude from the main cell body Specialized receptors for antigen uptake and efficient antigen-processing and -presenting pathways Express migratory, homing and lymphocyte-binding functions to assist in moving towards lymphoid tissues Distinct subsets of different origins found in blood, lymph and tissues—e.g. myeloid and plasmacytoid in blood; CD14+ or interstitial DCS, Langerhans cells and microglia, two self-renewing DC populations found in stratified squamous epithelium and parenchyma of the brain, respectively Markers include CD103, CD45, MHC-II, CD11c and Flt3		1. Immunomodulatory link for innate and adaptive immune systems as major APCs 2. DC are involved with development of immune tolerance in T cells by driving deselection of those responding to self-antigens in thymus 3. A class of DC known as follicular DC forms complexes with antigen and antibody at lymph nodes, serving as ‘reservoir’ for antibody, by providing sustained stimulation of the relevant B cells	2–3 days (FDC may last for months or years)	[39–43]
Eosinophils	Granulocytes, rich in cationic proteins, cytokines, chemokines and growth factors Rapidly release a variety of mediators without the need for de novo protein synthesis Cell surface markers include: IL-5Rα, CCR3, Siglec-8, EMR1, CD11b	1–4%	1. Involved in allergies and reactions to parasites 2. Phagocytic and APCs, although less efficient 3. Maintains epithelial barrier function 4. Orchestrates tissue remodelling events	8–12 days	[44–46]
Basophils	Granulocytes rich in IL-4 Express the high-affinity receptor for IgE, FcεRI Constitutively express MHC class II and costimulatory molecules such as CD40, CD80 and CD86	Rare, ~ 0.5%	1. Are primary source of IL-4 in vivo for generation of Th2 cells and boosting antibody response 2. Act as APCs, especially for soluble antigens of parasites 3. Mediate long-term allergic reactions such as asthma or skin allergies	1–2 days	[47, 48]
Mast cells	Similar to basophils; also produce IL-1 family	Found in tissues around blood vessels	1. Regulate innate and adaptive immune responses 2. Promote inflammatory reactions		[49, 50]
Natural killer cell	Granulocytes that constitutively express genes for perforin and granzymes in cytotoxic organelles for killing both viral infected and tumor cells, without prior antigen exposure Like B and T cells, NK cells establish a small pool of long-lived antigen- specific ‘memory’ cells after initial antigen encounter Molecular markers: Neural cell adhesion molecule CD56, CD4±, CD8− TCRαβlow, CD24−, CD44+, NK1.1+	5–15%	1. Directly bind and kill infected, foreign and cancer cells, especially those that would escape adaptive T cell response caused by MHC-I downregulation 2. Engage in immunomodulatory interactions with dendritic cells, macrophages, T cells and endothelial cells		[51–54]
Adaptive immune cells
Plasma or effector B cells	Express the B cell immunoreceptor Molecular markers: B220+, CD19+, CD79, CD43−, surface IgM+	2–5%	1. Produce antibody against foreign or non-self-antigens 2. Regulate diverse immune responses through their production of cytokines and other immune mediators		[54, 55]
CD8+ T cells	Constitutively express the T cell receptor–CD3 complex Cytotoxic T cells Molecular markers: CD4−, CD8+, TCRαβ+, CD24±	5–30%	Antigen-specific killing of infected, foreign or cancer cells		[54]
CD4+ T helper cells	Express the T cell receptor–CD3+ complex Proliferative in response to IL-2 Molecular markers: CD4+, CD8−, TCRαβ+, CD24±	20–40%	Depending on the subtype, the helper cells secrete cytokines, growth factors and chemokines that 1. Help B cells make antibodies 2. Stimulate cells involved in cell-mediated immunity 3. Induce macrophages to develop enhanced microbicide activity 4. Recruit neutrophils, eosinophils, and basophils to sites of infection and inflammation 5. Dampen the immune response		[54, 56, 57, 67]
Memory B or T cells	Can be B or T cell type Express the B cell receptor or T cell receptor–CD3+ complex, respectively			Months and years

MCOs microorganisms, WBC white blood cells