Opsonin is the umbrella term for various proteins. Opsonins occur, for example, as antibodies or complement factors and as such are involved in the body’s immune response. Opsonins play a role in numerous diseases, which include autoinflammatory diseases as well as infections.
What is opsonin?
In biology, opsonins are various proteins that are part of the immune system. They include antibodies and complement factors. Both are necessary for the defense against pathogens. The name opsonin is derived from the Greek “opsōneîn”, which means “to prepare for eating”: opsonins prepare microorganisms for phagocytes. Phagocytes are scavenger cells that can eliminate tissue, bacteria, fungi, parasites or viruses. Some of the opsonins act as markers (e.g. antibodies), while others help the phagocytes bind to the dangerous cells (e.g. fibronectin). Opsonins can be divided into three groups: Antibodies attach to antigens, thereby signaling to the immune system that a potential pest is present. Complement factors in turn react to the antigens. The third group of opsonins circulates freely in the blood.
Function, action, and roles
The antibodies of the immune system represent a variant of opsonins. Antibodies are soluble blood proteins that can bind to antigens. Antigens are structures on the surface of cells that provide information about the type of cell. The immune system uses antigens to recognize foreign bodies and identify pathogens. Antibodies mark the conspicuous objects and thus enable a defense response. The complement factors of the immune system also belong to the opsonins. The complement system consists of plasma proteins that are dissolved or cell-bound in the blood. They primarily participate in the immune response to microorganisms such as fungi, bacteria or parasites. For this purpose, the complement factors attach to the invader and cover its surface. Biology calls this process opsonization. Opsonization signals the danger of the opsonized object and stimulates phagocytes to ingest and digest it. One nonspecific opsonin is fibronectin. It is found in the extracellular matrix and is involved, for example, in tissue repair, cell migration and adhesion, and hemostasis. In the immune response, fibronectin plays a mediating role: it helps phagocytes bind to antigens. Another opsonin is C-reactive protein (CRP), which is one of the acute phase proteins: The body produces it in greater quantities when there is an acute infection or inflammation. CRP activates the complement system. PTX3 also performs a similar function – but the receptor reacts not only to various bacteria, fungi and viruses, but also to the body’s own cells that pose a threat. Cells that are severely damaged or carry a virus initiate their own destruction as soon as they recognize the risk and cannot eliminate it in any other way. This cell suicide is also known as apoptosis. PTX3 also targets such cells, helping to remove them from phagocytes before damaging cells spread.
Formation, occurrence, properties, and optimal levels
The body synthesizes the various opsonins in different organs. For example, the liver produces CRP. A single opsonin may consist of several hundred amino acids that combine to form a long chain. The sequence of amino acids within the chain is determined by the genetic code. Mutations can upset the sequence of amino acids and thus alter the structure of opsonins. Possible consequences are diseases of the immune system, autoinflammatory diseases or disorders in the synthesis of tissue. Doctors can detect some opsonins using blood tests. For example, testing can help reveal an inflammatory reaction that is not visible. The reference value for CRP is 10 mg/l for a healthy adult. If the measured value is higher, this is an indication of an acute infection or inflammatory reaction. Further examinations or other blood parameters such as disease-specific inflammatory markers may be able to provide the exact cause of abnormal opsonin values.
Diseases and disorders
The individual opsonins may be related to disease in different ways. A mutation in the FN1 gene alters the opsonin fibronectin, which promotes phagocyte binding to antigens. As a result, the X-type of Ehlers-Danlos syndrome may manifest. The clinical picture is characterized by a disorder of the connective tissue. It is characterized mainly by over-movement of the joints and over-extensibility of the skin. In addition, Ehlers-Danlos syndrome leads to changes in muscles, vessels, internal organs, tendons and ligaments. Since the syndrome affects numerous organ systems, the symptoms are very diverse: they include heart problems, premature osteoarthritis, intervertebral disc degeneration, soft and thin skin, frequent injuries, delayed motor development in children, abnormalities of the dentition and gums, mild or severe digestive disorders, neuralgia, migraine, eye diseases, and many other complaints and disorders. In addition, psychological symptoms such as unusual anxiety, depression, pain and sleep disorders often manifest. To diagnose Ehlers-Danlos syndrome, physicians must consider the clinical picture and also learn if any family members suffer from the rare disease. Although they normally work in favor of health, complement factors can cause direct harm to the human organism – when they get out of control and damage the body’s own tissues. This process occurs in rheumatoid arthritis or systemic lupus erythematosus, among others. The opsonin PTX3 appears to be involved in the immune response in a variety of circumstances. For example, it responds to influenza virus, appears more strongly in renal failure, and supports the defense against fungal infection by Aspergillus fumigatus. Furthermore, PTX3 is also involved in the inflammatory response in rheumatoid arthritis, SIRS, sepsis, and others.
