opsonin

Opsonization is a critical step in the immune response whereby opsonins bind to the surface of pathogens, apoptotic cells, or immune complexes, facilitating their recognition and engulfment by phagocytic cells expressing corresponding receptors (e.g., Fc receptors for IgG, complement receptors like CR1 for C3b). The main opsonins include IgG (particularly IgG1 and IgG3 subclasses), complement fragment C3b (and its degradation product iC3b), and to a lesser extent, C-reactive protein and mannose-binding lectin, which can also enhance phagocytosis via complement activation.

Mechanistically, IgG opsonizes pathogens by binding specific antigens via its Fab region, while its Fc region is recognized by Fc receptors (FcγR) on phagocytes, triggering engulfment. C3b, generated via classical, alternative, or lectin complement pathways, binds covalently to microbial surfaces and is recognized by complement receptor 1 (CR1) on phagocytes.

Clinical relevance: opsonization deficiencies predispose to recurrent bacterial infections. For example, patients with complement deficiencies (e.g., C3 deficiency) have increased susceptibility to encapsulated bacterial infections (e.g., Streptococcus pneumoniae, Neisseria meningitidis) due to impaired opsonophagocytosis. Similarly, asplenic patients (post-splenectomy) are at risk because the spleen is a major site for antibody production and clearance of opsonized bacteria. In sickle cell disease, functional asplenia leads to poor opsonization and susceptibility to encapsulated organisms, necessitating vaccination against S. pneumoniae, N. meningitidis, and Haemophilus influenzae type b.

Opsonins are also relevant in autoimmune hemolytic anemia, where autoantibodies (IgG) opsonize red blood cells, leading to their destruction by splenic macrophages (extravascular hemolysis) — the basis of the direct Coombs test.