Dr Holt is recognised internationally for his pioneering studies on the identification and characterisation of the cellular interactions responsible for regulating immunity in the airways. He was the first: (a) to discover the respiratory mucosa! dendritic cell network and its 'gatekeeper' function in antigen surveillance and induction of primary immunity in the lung; (b) to delineate an opposing inhibitory role for endogenous tissue macrophages in controlling T cell memory at that site. His work has profound implications for the understanding of susceptibility to asthma, immunisation with and tolerance induction to antigens delivered via the respiratory route and early intervention in allergic diseases.