opson

Opso n, short for opsonin, is a molecule that binds to a pathogen or other foreign particle and marks it for attack by phagocytes, thereby increasing the efficiency of phagocytosis. In humans, the most important opsonins are antibodies (immunoglobulins) and complement components such as C3b. Other proteins, including collectins like mannose-binding lectin, can also function as opsonins in innate immunity.

Mechanism and function: Opsonins coat the surface of microbes or debris. Phagocytes display receptors that recognize these coatings, such as Fc receptors for the constant region of antibodies and complement receptors for C3b. Binding triggers the engulfment of the opsonized particle and its destruction within the phagolysosome. This process enhances clearance of bacteria, fungi, and apoptotic cells and is a central aspect of both humoral and innate immune responses.

Types and examples: Within humans, immunoglobulin G (IgG) and IgM can act as opsonins, as can the complement fragment C3b. Collectins and other acute-phase proteins also contribute to opsonization in specific tissues and contexts. The relative importance of each opsonin can vary with the pathogen and the site of infection.

Clinical relevance: Defects in opsonization or opsonin production can lead to increased susceptibility to infections, particularly with encapsulated bacteria such as Streptococcus pneumoniae. Therapies that enhance opsonization, including intravenous immunoglobulin (IVIG) and vaccines that elicit opsonizing antibodies, play important roles in prevention and treatment.