Opsonization

Opsonization is the process by which pathogens are marked for ingestion and destruction by phagocytes. Opsonins are molecules that bind to the surface of pathogens, making them more recognizable to phagocytic cells. The best known opsonins are antibodies, particularly immunoglobulin G (IgG), and complement fragments such as C3b. Other components, including C4b and certain acute-phase proteins such as C-reactive protein (CRP) and mannose-binding lectin (MBL), can also function as opsonins under specific conditions.

Phagocytes possess receptors for opsonin-bound targets: Fc receptors for the Fc region of antibodies and complement receptors for C3b and related fragments. When a microbe is opsonized, these receptors trigger enhanced binding, engulfment, and intracellular killing through phagocytosis. Opsonization can be initiated via the classical pathway of complement, which is antibody-dependent, or via the lectin and alternative pathways, which can deposit C3b on microbial surfaces even in the absence of specific antibodies. CRP and MBL can opsonize microbes by binding to their surface and then engaging phagocyte receptors.

Pathogens may evade opsonization by expressing capsules that prevent antibody or complement deposition, or by interfering with complement activation. Encapsulated bacteria such as Streptococcus pneumoniae and Neisseria meningitidis are classic examples of organisms particularly affected by opsonization.

Clinically, deficiencies in antibodies, complement proteins, or phagocyte receptors that impair opsonization can lead to increased susceptibility to infections, especially with encapsulated bacteria, highlighting opsonization as a key mechanism in humoral and innate immunity.