An opsonin is any molecule that acts as a binding enhancer for the process of phagocytosis, for example, by coating the negatively-charged molecules on the membrane. Both the membrane of a phagocytising cell, as well as its target, have a negative charge (Zeta-potential), making it difficult for the two cells to come close together. During the process of opsonization, antigens are bound by antibody and/or complement molecules.