Figure 2: Complex crosstalk between inflammatory cells, neuropeptides, neurotrophins, and cytokines in allergic rhinitis. Substance P (SP) and calcitonin gene-related peptide (CGRP) act synergistically (along with vasoactive intestinal peptide (VIP)) to promote vasodilation and plasma extravasation causing nasal congestion. SP and CGRP also activate monocytes to release proinflammatory cytokines and promote degranulation of primed mast cells contributing to early-phase allergic response. Nerve growth factor (NGF) promotes the production and release of SP and CGRP and also promotes the survival of eosinophils and mast cells hence prolonging inflammatory response. CGRP: calcitonin gene-related peptide, SP: substance P, VIP: vasoactive intestinal peptide, NKA: neurokinin A, NGF: nerve growth factor, TNF-α: tumour necrosis factor alpha, IL-1: interleukin 1, IL-4: interleukin 4, IL-6: interleukin 6, IL-10: interleukin 10. Images courtesy of Dr P. K. Smith.