Which molecule commonly acts as an opsonin?

Which molecule commonly acts as an opsonin?

• A. IL-6
• B. IgA
• C. IgM
• D. IgG

Answer: (D)

Opsonization is the process by which a pathogen is marked for phagocytosis, usually by coating it with molecules that phagocytes recognize. The most common opsonins are antibodies that, when bound to a pathogen, present an Fc region that phagocytes’ Fc receptors can bind, promoting uptake and destruction. Among the options, IgG is the classic and most efficient opsonin in the bloodstream because its Fc region strongly interacts with Fc gamma receptors on neutrophils and macrophages, triggering phagocytosis and microbial killing. IgG1 and IgG3 are particularly effective in this role. IgM can also promote opsonization indirectly by activating complement and leading to C3b deposition on a surface, which acts as an opsonin, but as a direct, widely acting opsonin in circulation, IgG is the best answer. IgA mainly functions at mucosal surfaces to block adherence and neutralize pathogens rather than serving as the principal opsonin in circulation. IL-6 is a cytokine involved in inflammation, not an opsonin.

Opsonization is the process by which a pathogen is marked for phagocytosis, usually by coating it with molecules that phagocytes recognize. The most common opsonins are antibodies that, when bound to a pathogen, present an Fc region that phagocytes’ Fc receptors can bind, promoting uptake and destruction. Among the options, IgG is the classic and most efficient opsonin in the bloodstream because its Fc region strongly interacts with Fc gamma receptors on neutrophils and macrophages, triggering phagocytosis and microbial killing. IgG1 and IgG3 are particularly effective in this role.

IgM can also promote opsonization indirectly by activating complement and leading to C3b deposition on a surface, which acts as an opsonin, but as a direct, widely acting opsonin in circulation, IgG is the best answer. IgA mainly functions at mucosal surfaces to block adherence and neutralize pathogens rather than serving as the principal opsonin in circulation. IL-6 is a cytokine involved in inflammation, not an opsonin.