An opsonin (from the Greek opsoneîn, to prepare for eating) is any molecule that enhances phagocytosis by marking an antigen for an immune response (i.e., causes the phagocyte to "relish" the marked cell). However, the term is usually used in reference to molecules that act as binding enhancers for the process of phagocytosis, especially antibodies, which coat the negatively charged molecules on the membrane. Molecules that activate the complement system are also considered opsonins. Phagocytic cells do not have an Fc receptor for immunoglobulin M (IgM), making IgM ineffective in assisting phagocytosis. However, IgM is extremely efficient at activating complement and is, therefore, considered an opsonin. Opsonins can also refer to molecules that target a cell for destruction through the action of natural killer (NK) cells.