Opsonization: Definition, Importance, Associated Problems, and Substances that Act as Opsonins

It involves the binding of an opsonin to a receptor on the pathogen’s cell membrane.

A pathogen is a virus, fungus, or bacterium capable of causing disease in the body.

The antibodies are proteins that exist in body fluids and are used as a detection device and a response from the immune system.

In this opsonization process in which pathogens become coated with opsonin, the pathogen is marked for destruction by the immune system.


The process of ingesting and killing a pathogen is called “phagocytosis.”

The phagocytes ingest pathogens and then kill the exposure to toxic chemicals.

The chemicals are stored in small membrane-bound vesicles within phagocytes, and these are activated to open when a phagocyte ingests a pathogen.


The efficiency of phagocytosis is significantly increased when microbes are opsonized by specific proteins (opsonins) for which phagocytes express high-affinity receptors.

Opsonin receptors promote phagocytosis of microbes coated with various proteins and emit signals that activate phagocytes.

Antibody-dependent cellular cytotoxicity

Opsonization also leads to pathogen death in a second mechanism known as “antibody-dependent cellular cytotoxicity,” in which immune cells directly kill pathogens without ingesting them.

Antibodies are proteins that exist in body fluids and are used as the immune system’s detection and response device.

The antibodies act like opsonins and then activate immune cells called ‘granulocytes’ during this process.

Granulocytes are a type of lymphocyte. These cells are part of the immune system and are involved in several different immune reactions.

The granulocytes then release toxic chemicals into the environment around the pathogens to destroy them.

In addition to killing pathogens, this procedure also causes tissue damage through inflammation.

Importance of opsonization

Opsonization of pathogens, such as bacteria and viruses, is essential since both immune and pathogenic cells are negatively charged.

It is not easy for a cell to move close enough to a pathogen to begin ingestion or direct death.

Opsonization solves this problem as they have receptors that identify and bind to protein molecules on immune cells.

Thus, when a pathogen has become coated with opsonins, receptors for the opsonins can bind, bringing cells closer to pathogens for easy ingestion or direct elimination.

Substances that act as opsonins

Several substances work as opsonins; all of these are proteins active in the immune system.

The main opsonins are immunoglobulin G antibodies, complement proteins, and specific plasma lectins, which are recognized by specific receptors on leukocytes.

The two types of antibodies known as immunoglobulin G and immunoglobulin A are opsonins.

  • Immunoglobulin G: is active in the blood.
  • Immunoglobulin A is active on mucosal surfaces, such as the respiratory tract, the urogenital system, and the intestine.

One of the most efficient systems for opsonizing particles is to coat the particles with antibodies such as immunoglobulin G, which are called specific opsonins and are recognized by a high-affinity receptor on phagocytes, called FcγRI.

Many proteins that act in the complement system are also opsonins. The complement system is a cascade of reactions between different proteins.

Components of the complement system, especially fragments of the complement protein C3, are potent opsonins.

These fragments bind to microbes, and phagocytes express a receptor, called the complement receptor type 1, that recognizes the degradation products of C3.

Several plasma proteins, including lectin, fibronectin, fibrinogen, and C-reactive protein, can also coat microbes and are recognized by receptors on phagocytes.

The result is the opsonization of pathogens and their direct destruction through forming a protein complex that penetrates the orifice of the bacterial cell wall.

Problems in opsonization

Several types of inherited genetic diseases can cause opsonization problems.

People with diseases that cause deficiencies in the complement system are more prone to infections, mainly bacterial infections.

People who have diseases that affect B lymphocytes, the cells that produce antibodies, are also prone to infection to a greater extent.

People with these immune deficiencies have different levels of risk for severe and even fatal infections with pathogens that would not cause disease in healthy people.