Nature's 'nanocarriers' are viruses, which protect, target, and deliver their active agents to cells. Viruses are found to have both quasi-spherical and filamentous shapes, and how such particles are transported in the body and interact with various cells, especially immune cells is a question of general importance. Synthetic, rather than natural, polymers are being made to assemble in water into similar shapes as viruses in order to study interactions and perhaps develop applications. Such studies have motivated the question: how do the main policing cells of the immune system, called macrophages, distinguish "foreign" objects and cells from "self" cells? A two-step procedure for recognizing intruders helps avoid misdirected attacks. In the first step, macrophages ensnare and begin swallowing objects studded with antibody molecules, which usually bind to interlopers. But the antibodies are sloppy, sometimes attaching to the body’s own cells. So before a macrophage engulfs its target, it also checks for a second form of identification, a particular membrane protein called CD47. If a “self” version of CD47 is present, the macrophage is turned off, through a specific blockade of the macrophage's 'nano-motors' that are used to eat objects. Some viruses might be exploiting such mechanisms to evade the immune system, thus motivating ongoing studies of CD47-decorated particles of various sizes and shapes.