Opsoniny

Opsoniny are molecules that bind to microbes or other particles and mark them for ingestion by phagocytes. Opsonization enhances phagocytosis by promoting the attachment of the microbe to receptors on neutrophils and macrophages, thereby increasing the efficiency of clearance from the body. Opsonins can be antibodies, components of the complement system, or other soluble molecules that recognize microbial surfaces.

Common opsonins include antibodies, especially immunoglobulin G (IgG) and, to a lesser extent, immunoglobulin A (IgA), which are recognized by Fc receptors on phagocytes. Complement fragments such as C3b, iC3b, and C3d bound to a microbe engage complement receptors on phagocytes (for example CR1 and CR3). Lectin pathway components like mannose-binding lectin (MBL) can also act as opsonins by coating pathogens and promoting complement deposition. C-reactive protein and other collectins can function as opsonins by binding to microbial surfaces and facilitating receptor-mediated uptake.

The process of opsonization can occur via antibody-dependent mechanisms (the classical pathway) or via complement pathways that generate opsonizing fragments independently of antibodies. By linking pathogens to Fc receptors or complement receptors, opsonins enhance binding, phagocytosis, and intracellular killing, contributing to host defense against bacteria, fungi, and other microbes.

Defects in opsonization, whether from antibody deficiencies or complement defects, can lead to increased susceptibility to infections. Therapeutic approaches that enhance opsonization, such as intravenous immunoglobulin or complement-targeted therapies, are used in certain immunodeficiencies and clinical conditions.