Opsoninen

Opsoninen is the Finnish adjective used for substances that act as opsonins, i.e., molecules that bind to the surface of pathogens and promote their recognition and removal by phagocytic cells. The process driven by these molecules is known as opsonization, a key mechanism in innate and adaptive immunity that enhances phagocytosis and clearance of microbes.

Mechanism and receptors: Opsonins coat microbial surfaces, enabling phagocytes to detect and ingest them more efficiently. Phagocytes express receptors that recognize bound opsonins, including Fc receptors for antibodies and complement receptors (such as CR1 and CR3) for complement fragments. Engagement of these receptors stimulates engulfment, intracellular killing, and inflammatory signaling.

Major examples: The most important opsonins are antibodies, particularly immunoglobulin G (IgG) and, in some contexts, IgM, which bind to pathogens and interact with Fc receptors on phagocytes. Complement components that function as opsonins, notably C3b (and its fragments iC3b, C4b), decorate microbial surfaces and promote uptake via complement receptors. Innate opsonins such as mannose-binding lectin (MBL) and acute-phase proteins like C-reactive protein (CRP) can also facilitate opsonization by binding microbes and enhancing receptor-mediated phagocytosis.

Clinical relevance: Opsonization is essential for effective defense against many pathogens, especially encapsulated bacteria such as Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis. Deficiencies in antibodies, complement components, or opsonin receptors can increase susceptibility to infection. Vaccination and antibody-based therapies work, in part, by enhancing opsonization and subsequent phagocytosis.