CD200 and experimental autoimmune encephalomyelitis: When CD200 interacts with its cognate receptor CD200R1 on myeloid cells, it delivers a selective inhibitory signal.1 The CD200-CD200R1 checkpoint is involved in dampening microglial activation in experimental autoimmune encephalomyelitis,2 maintaining alveolar macrophage tolerogenic properties during lung infection5 and preventing red pulp macrophage expansion.2 The effects of the CD200-CD200R1 pathway on monocytes and monopoiesis are unclear as are the mechanisms of increased tissue macrophage numbers in CD200-deficient mice that is seen in a variety of contexts.2