Various Flashcards
What is opsonization?
Antibody opsonization is the process by which a pathogen is marked for ingestion and destruction by a phagocyte. Opsonization involves the binding of an opsonin, e.g., antibody, to an epitope on an antigen.[1] After opsonin binds to the membrane, phagocytes are attracted to the pathogen. The Fab portion of the antibody binds to the antigen, whereas the Fc portion of the antibody binds to an Fc receptor on the phagocyte, facilitating phagocytosis.[2] The receptor-opsonin complex can also create byproducts like C3b and C4b which are important components of the complement system. These components are deposited on the cell surface of the pathogen and aid in its destruction.
What is the complement system?
The complement system helps or “complements” the ability of antibodies and phagocytic cells to clear pathogens from an organism. It is part of the immune system called the innate immune system[1] that is not adaptable and does not change over the course of an individual’s lifetime. However, it can be recruited and brought into action by the adaptive immune system.
The complement system consists of a number of small proteins found in the blood, in general synthesized by the liver, and normally circulating as inactive precursors (pro-proteins). When stimulated by one of several triggers, proteases in the system cleave specific proteins to release cytokines and initiate an amplifying cascade of further cleavages. The end-result of this activation cascade is massive amplification of the response and activation of the cell-killing membrane attack complex. Over 30 proteins and protein fragments make up the complement system, including serum proteins, serosal proteins, and cell membrane receptors. They account for about 5% of the globulin fraction of blood serum and can serve as opsonins.
Three biochemical pathways activate the complement system: the classical complement pathway, the alternative complement pathway, and the lectin pathway.
What are opsonins?
An opsonin (from the Greek opsonin, 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 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 (NK) cells.
