part of hematopoietic system reticuloendothelial system A diffuse system of cells of varying lineage that include especially the macrophages and the phagocytic endothelial cells lining blood sinuses and that were originally grouped together because of their supposed phagocytic properties based on their ability to take up the vital dye trypan blue. BTO:0001174 BrendaTissueOBO lymphoreticular system