Macropinocytosis has received increasing attention in recent years for its 
various roles in nutrient acquisition, immune surveillance, and virus and cancer 
pathologies. In most cases macropinocytosis is initiated by the sudden increase 
in an external stimulus such as a growth factor. This "induced" form of 
macropinocytosis has been the subject of much of the work addressing its 
mechanism and function over the years. An alternative, "constitutive" form of 
macropinocytosis restricted to primary innate immune cells also exists, although 
its mechanism has remained severely understudied. This mini-review focuses on 
the very recent advances that have shed new light on the initiation, formation 
and functional relevance of constitutive macropinocytosis in primary innate 
immune cells. An emphasis is placed on how this new understanding of 
constitutive macropinocytosis is helping to define the sentinel function of 
innate immune cells including polarized macrophages and dendritic cells.