Opsonin

Opsonins are occurring in the body proteins, which are a part of the immune response. Opsonins either antibody or complement may be to facilitate the linking of phagocytes in bacterial cells or other microorganisms, by acting as a bridge between the two cells. In this context one also speaks of the opsonization of the target cell. Phagocytes can bind by itself directly to bacteria or other cells with antigen character to a certain extent. However, the binding is by an activated complement - enhanced by the complement to function as a bridge between the bacterium and the corresponding complement receptor of phagocytes - a plasma protein, which is the part of the non-specific humoral immune response.

In organisms that may enable only small amounts of complement, it is alternatively or additionally opsonized with antibodies. The binding principle here is similar: the antibody is linked via its Fc fragment with Fc receptor of phagocytes and its Fab fragments with the corresponding epitope of the antigenic microorganism. Phagocytosis by opsonization of the bound organism, and will ultimately be secured, the cell lysis may be initiated.