CCR2 and infection: Though macrophage numbers in Ccr2–/– tissues remain considerably lower than seen in WT mice, there are two explanations for the subtle increase: (1) loss of CCR2 may not completely abrogate monocyte recruitment, as monocytes could be migrating via other chemokine receptors, or (2) tissue-resident macrophages, or even tissue-resident hematopoietic stem cells, could undergo emergency hematopoiesis and proliferate in response to infection (Boettcher and Manz, 2017).