Opsonization is a process in which pathogens are coated with a substance called an opsonin, marking the pathogen out for destruction by the immune system. Once a pathogen has been opsonized, it is killed via one of two mechanisms. The pathogen may be ingested and killed by an immune cell, or killed directly without ingestion.

The process of killing and ingesting a pathogen is called phagocytosis. Cells called phagocytes ingest the pathogens and then kill them by exposing them to toxic chemicals. The chemicals are stored in small membrane-bound parcels within the phagocytes, and these parcels are triggered to open when a phagocyte ingests a pathogen.

Opsonization also leads to pathogen death in a second mechanism called antibody-dependent cellular cytotoxicity, in which immune cells directly kill pathogens without ingesting them. In this process, antibodies act as opsonins, and then trigger immune cells called granulocytes. These cells then release toxic chemicals into the environment around the pathogens to kill them. In addition to killing pathogens, this process also causes tissue damage via inflammation.

Opsonization of pathogens such as bacteria and viruses is necessary because both immune cells and pathogen cells are negatively charged. This means it is difficult for a cell to move close enough to a pathogen to initiate ingestion or direct killing. The evolution of opsonins solves this problem because they have receptors which recognize and bind to protein molecules on immune cells. Therefore, when a pathogen has been coated in opsonins, the receptors on the opsonins can bind immune cells, bringing the cells close enough to the pathogens to enable ingestion or direct killing.

There are several different substances which may act as opsonins; all of these are proteins which are active in the immune system. Two antibody types called IgG and IgA are both opsonins. IgG is active in blood and tissues, and IgA is active in mucosal surfaces such as the airways, urogenital system, and gut. Several proteins which act in the complement system are also opsonins. The complement system is a cascade of reactions between a number of different proteins. The end result of the cascade is opsonization of pathogens, as well as direct pathogen killing via the formation of a protein complex which punctures holes in bacterial cell walls.

Several types of inherited genetic disease can cause defects in opsonization. For example, people with diseases which cause deficiencies in the complement system are more susceptible to infections, particularly bacterial infections. Diseases which affect B lymphocytes, the cells which produce antibodies, also lead to increased susceptibility to infection. People with these immune deficiencies have varying levels of risk to serious and even fatal infections with pathogens which would not cause disease in healthy people.