Table 1.

A summary of key cell subsets, their role in allergic rhinitis (AR), and allergen immunotherapy (AIT)-induced effects on their responses. EC, epithelial cells; DC2, dendritic cells of type 2; ILC2s, group 2 innate lymphoid cells; T_H2, T helper type 2 cell; T_FH2, type 2 follicular helper T cells

Stage of allergy	Cell subset	Key functional molecules	Role in allergic rhinitis	Effects of allergen immunotherapy
Sensitization	EC	CCL20 [23, 24•] TSLP, IL-25, IL-33 [26, 28] RNA GAS5 [35]	DC recruitment [24•] Induction of ILC2 [29] and DC2 [27] cell subsets Promotion of TH2 [30] and inhibition of TH1 [35] polarization Promotion of mast cell degranulation [31]	Restoration of EC integrity in murine models [81]
	DC2	MHC II CD80/CD86 [32]	TH2 polarization Antigen presentation [32]	Promotion of DCreg polarization [27, 82] Increased FcεRI expression [46•]
	TFH2	Surface CD40 secreted IL-4, IL-13, IL-21 [36, 39]	Induction of B cell ε-germline transcription, differentiation and affinity maturation and sIgE production [39, 41]	Decreased circulating numbers [38, 83] Restored function of suppressive counterparts TFR cells [84]
	B cells	IgE [42]	Mast cell and basophil sensitization [42] Antigen presentation [85••]	Induction of IgG+ B cells [39, 47, 86–88•] Induction of IL-10+Bregs [87–90] Reduced CD23 expression [85••]
Early phase responses	Basophils	Histamine, tryptase, cysteinyl leukotrienes, PGD2 [43, 44, 50, 51]	Promotion of TH2 polarization [33] Activation of ILC2s [51] Increasing vascular permeability and immune cell influx [50] Induction of symptoms: itching, sneezing, rhinorrhea [44]	Reduced infiltration [91–94] Decreased sensitivity and degranulation [95, 96••, 97, 98]
	Mast cells
Late phase responses	TH2	IL-4, IL-5, IL-9, IL-13 [44, 50, 64, 67, 99]	Induction of B cell ε-germline transcription, differentiation and affinity maturation [41] Downregulation of tight junctions of nasal epithelium [50] Upregulation of endothelial adhesion molecules [61] Promotion of basophil and mast cell differentiation [67] Eosinophil recruitment and survival [62, 63]	Induction of iTregs [100–103] and nTregs [103–106] Reduced TH2 and increased TH1 frequencies and corresponding cytokines [100, 107–112]
	ILC2s			Reduced circulating frequencies [80, 113•]
	Eosinophils	LTC4, PGE2, EDN, ECP [66]	Damage of the nasal epithelium [65]	Reduced infiltration [92, 104, 107, 114, 115]
	Neutrophils	MHC II [74]	Antigen presentation [74]	Reduced activation (157)