Opsonization is the process by which particles such as bacteria are coated with host molecules that promote their recognition and ingestion by phagocytic cells.
Explanation
The immune system uses opsonization to mark microbes and other foreign particles for destruction. Opsonins include antibodies, particularly IgG subclasses, and complement components such as C3b and C4b. These molecules bind to antigens on the surface of a microbe and provide a handle for phagocyte receptors. Neutrophils and macrophages express Fc receptors that bind the Fc region of IgG and complement receptors that recognize bound C3 fragments. When a pathogen is opsonized, the affinity and avidity between the phagocyte and the microbe increase, triggering actin rearrangement, engulfment and the formation of a phagosome. Complement activation also generates C5a, a chemoattractant that recruits phagocytes to the site of infection. Opsonization bridges innate and adaptive immunity by linking antibody specificity to innate effector mechanisms.
Roles and clinical significance
Opsonization is crucial for clearing encapsulated bacteria such as Streptococcus pneumoniae and Neisseria meningitidis, whose polysaccharide capsules resist direct phagocytosis. IgG antibodies specific for capsular antigens coat these bacteria, enabling neutrophils to ingest and kill them. Complement deficiencies that impair C3 deposition or Fc receptor defects can lead to recurrent infections due to inadequate opsonization. Conversely, some pathogens evade opsonization by expressing proteins that bind the Fc region of antibodies (protein A in Staphylococcus aureus) or by inhibiting complement activation. Therapeutic monoclonal antibodies used in cancer and autoimmune diseases often act through opsonization and antibody-dependent cellular phagocytosis. Vaccines that elicit opsonizing antibodies are effective because they enhance clearance of the target pathogen.
Opsonization illustrates how the immune system labels potential threats and directs effector cells to remove them. Efficient opsonization depends on the coordinated function of antibodies, complement and phagocyte receptors.
Related Terms: Phagocytosis, Complement system, Antibody, Fc receptor, Innate immunity