Opsonization

What is opsonization?

Opsonization is a process that helps your immune system recognize and destroy harmful invaders (germs/pathogens) and dead or damaged cells. Your immune system is like a security team that monitors your body, looking for anything that could harm you or make you sick.

The different parts of your immune system — organs, cells, proteins and chemicals — work together. Some parts patrol your body, looking for invaders. Others destroy things that could harm your body. Opsonins are proteins that coat pathogens (like viruses or bacteria) and mark them so other cells of your immune system can destroy them. Opsonization also marks dead or dying cells, infected cells and tumor cells.

What’s an opsonin?

The word “opsonin” is used for many different types of proteins that coat cells and pathogens to help special immune cells (phagocytes) recognize and digest them. Some float freely in your blood or lymph and some are created from other proteins when your immune system is activated. Many of them, like antibodies, perform other jobs but can work like an opsonin.

Phagocytes attach to the opsonins like a bridge, and use it to swallow (ingest) the object and destroy it. Because of this, opsonins are known as bridging molecules.

What is phagocytosis?

Phagocytosis is the process your body uses to destroy things that are harmful or unnecessary. These could be germs (pathogens), foreign material or your own cells that need to be destroyed, including:

• Viruses, bacteria, fungi or other pathogens.
• Infected cells.
• Dead, aging or damaged cells.
• Tumor cells.
• Bits of cells, or cellular debris.
• Inactive or less active synapses (the spaces between nerve cells).

The word “phagocytosis” means “cell eating.” Phagocytes swallow or “eat” cells and pathogens, then break them apart so your body can get rid of them. Macrophages, dendritic cells, neutrophils and natural killer (NK) cells all use opsonization to help find and get rid of things that are harmful or unnecessary.

Phagocytic signals

Phagocytes use different signals to know whether to swallow a cell. These include:

• Find-me signals. These are proteins or other substances that help phagocytes find cells or pathogens that they should destroy.
• Eat-me or don’t-eat-me signals. These tell phagocytes to swallow or not swallow something.
• Opsonins. These are proteins that attach to the outside of cells or pathogens to tell phagocytes to swallow and destroy them. They also make it easier for phagocytes to attach (bind) to whatever they need to destroy, like a bridge.
• Negative opsonins. These discourage or make it harder for phagocytes to attach to something to destroy it.

What are examples of opsonization?

There are dozens of opsonins that your body uses to identify different targets to get rid of. Some examples include:

• Antibodies. Antibodies are a part of your immune system that fit onto specific parts of pathogens (antigens) like a lock and key. Immunoglobulin G (IgG) and immunoglobulin M (IgM) are antibodies that also work as opsonins.
• Complement factors. Complement factors are a group of proteins that work together to help (or complement) your immune system. Different complement proteins do different jobs to bring phagocytes to cells they need to destroy. C1q, C3, C3b and C4b are all opsonins.
• Phosphatidylserine-binding opsonins. Phosphatidylserine is an eat-me signal that moves from the inside to the outside of cells when they’re dying (apoptosis). Many types of opsonins can attach to it, including Annexin A1, Apolipoprotein H, Del-1, Gas6 and protein S.
• Other opsonins. Other proteins that work as opsonins include calreticulin, fibronectin, ficolins (pattern recognition proteins), mannose-binding lectin, milk fat globulin (lactadherin) and pentraxins.

What does opsonization do in phagocytosis?

Phagocytes eat pathogens and cells your body doesn’t need (phagocytosis) so your body can get rid of them. Opsonins coat the outside of cells and harmful invaders, like seasoning that makes things look more delicious to phagocytes. (In fact, the word “opsonization” is from a Greek word meaning “to make tasty.”)

Phagocytes are drawn to the coating and stick to it using parts called receptors. In that way, opsonins are also like the fork that helps the phagocytes grab the tasty food. Then, they pull it inside and break it down (digest it).

How does opsonization happen?

Many different processes can cause opsonization, including:

• Innate immune response. Proteins like C3 recognize parts of bacteria, viruses or fungi and latch on to them (bind).
• Adaptive immune response. Antibodies recognize antigens on bacteria, viruses or other harmful invaders they’ve seen before. They fit onto them like a lock and key, making the antigen-antibody complex. Complement proteins bind to the complex and coat the pathogen or cell with additional opsonins.
• Eat-me signals. When cells are damaged or dying, they send out signals that tell your immune system to destroy them. Opsonins recognize them and bind to them.

These processes have many steps that work together to activate your immune system.

Why is opsonization important?

Pathogens hide from or avoid cells of your immune system to survive. Opsonization helps phagocytes find and attach to pathogens to destroy them. Your cells use opsonization as a way to signal to your immune system that they’re infected, damaged or dying. This lets your body know to get rid of those cells.

Can problems with opsonization make me sick?

Genetic disorders, and sometimes drug reactions and other causes, can lead to phagocytosis disorders. Compliment deficiency and other immune system disorders can change how your body identifies what’s harmful and what’s not. For instance:

• Your phagocytes might not be able to destroy things that could be harmful, like damaged cells, bacteria or viruses.
• Your phagocytes might destroy things they shouldn’t, like healthy cells.
• Either of these issues can lead to autoimmune disorders, like lupus, or neurological diseases, like Alzheimer’s. They can put you at higher risk for infections or cancer.

Can opsonization be used to treat diseases?

Experts are researching ways to use opsonization to treat diseases. They hope to develop ways to help your immune system by opsonizing harmful substances or preventing opsonization of normal cells. This research could be useful when developing treatments for:

• Cancer.
• Infectious diseases, especially certain bacterial infections.
• Autoimmune disorders.

Researchers also use opsonization to test how well vaccines and antibody treatments might work. These tests are called opsonophagocytic killing assays (OPKA).