Question
Opsonization
Answer
Opsonization is the process by which a microorganism (or other particle) is coated with specific molecules (opsonins) that enhance its recognition and uptake (phagocytosis) by phagocytic cells (neutrophils, macrophages) — it dramatically increases the efficiency of phagocytosis, especially of encapsulated organisms that otherwise resist ingestion.
Major opsonins:
- IgG antibody — the Fab portion binds specific antigen on the microbial surface, while the Fc portion is recognized by Fc receptors (FcγR) on the phagocyte surface, promoting attachment and engulfment.
- Complement component C3b (and its inactive fragment iC3b) — deposited on the microbial surface following complement activation (classical, alternative, or lectin pathway), and recognized by complement receptors (CR1, CR3) on phagocytes.
Mechanism: opsonin binding effectively “tags” the pathogen, bridging it to the phagocyte via specific receptors, which triggers receptor clustering, cytoskeletal rearrangement, and engulfment of the particle into a phagosome, which then fuses with a lysosome for intracellular killing and digestion.
Clinical significance: opsonization is particularly critical for clearing encapsulated bacteria (e.g., Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis), whose polysaccharide capsule otherwise resists direct phagocytic recognition; deficiencies in antibody production (e.g., hypogammaglobulinaemia) or complement (e.g., C3 deficiency) or splenic function (asplenia — the spleen is a major site of opsonin-dependent clearance) predispose to overwhelming infection with these organisms.