A great deal of recent research has identified chemoattractants and cellular activators responsible for neutrophil trafficking into inflamed tissues, as well as for lymphocyte homing to secondary lymphoid organs in the steady state and into foci of chronic inflammation (1–5). Considerably less is known about the molecules regulating the trafficking of monocytes, particularly the constitutive trafficking of monocytes through tissues in health and the recruitment of monocytes to lymph nodes in disease. Two articles in this issue of The Journal of Experimental Medicine (6, 7) and one in a recent issue (8) shed some light on this subject and also prompt some questions for future investigation.
Under steady-state conditions in mice about half of the circulating monocytes leave the bloodstream each day (9, 10). Effete monocytes are destroyed in the spleen, but a...
