opsonina

Opsonin (plural opsonins) is a molecule in the immune system that binds to microorganisms or particles, tagging them for attack by phagocytes. The process is called opsonization and increases the efficiency of phagocytosis by linking the microbe to phagocyte receptors. Opsonins can be antibodies, especially IgG, and components of the complement system, notably C3b, as well as other plasma proteins such as C-reactive protein and mannose-binding lectin.

Mechanism: Phagocytes express receptors for the Fc region of antibodies (Fc receptors) and for complement receptors that recognize opsonins. When a microbe is coated with an opsonin, the phagocyte binds via these receptors and engulfs the particle into a phagosome, which then fuses with lysosomes to destroy the pathogen. Opsonization also enhances intracellular killing and can modulate inflammatory responses.

Sources and types: Antibody-based opsonins include IgG and, to a lesser extent, IgM. Complement-derived opsonins include C3b and C4b. Non-immunoglobulin opsonins include C-reactive protein, serum amyloid P, and mannose-binding lectin. Opsonins can arise from humoral immune responses or acute-phase reactions and are produced during infection or inflammation.

Clinical relevance: Defects in opsonization—due to antibody deficiencies, complement deficiencies, splenic dysfunction, or impaired production of acute-phase proteins—raise susceptibility to bacterial infections, especially encapsulated organisms. Treatments such as intravenous immunoglobulin therapy provide polyclonal antibodies to support opsonization in certain immunodeficiencies. Understanding opsonization informs vaccine design and antimicrobial strategies.