High Quality Content by WIKIPEDIA articles! An opsonin is any molecule that targets an antigen for an immune response. However, the term is usually used in reference to molecules that act as a binding enhancer 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 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 cells. Only opsonins that enhance phagocytic activity will be further described in this article.