Innate and adaptive immunity work concertedly in vertebrates to restore homeostasis following pathogen invasion or other insults. Like all homeostatic circuits, immunity relies on an integrated system of sensors, transducers and effectors that can be analysed in cellular or molecular terms. At the cellular level, T and B lymphocytes act as an effector arm of immunity that is mobilised in response to signals transduced by innate immune cells that detect a given insult. These innate cells are spread around the body and include dendritic cells (DCs), the chief immune sensors of pathogen invasion. At the molecular level, DCs possess receptors that directly sense pathogen presence and tissue damage and that signal to control antigen presentation or to regulate a plethora of genes encoding effector proteins that regulate immunity. To what extent the complement of DCs in tissues is sufficient to detect infection and drive immunity remains unclear. The lecture will focus on understanding how the numbers of tissue DCs are regulated in homeostasis and during infection.