Which IgG subclasses promote opsonization and Fc receptor engagement, helping clear infections at mucosal surfaces?

• A. IgG1 and IgG3 promote opsonization and Fc receptor engagement.
• B. IgG2 and IgG4 promote opsonization.
• C. IgA is the primary mucosal immunoglobulin in serum.
• D. IgM is the primary mediator of long-term memory.

Answer: (A)

The key idea is how different IgG subclasses engage the immune system through Fc receptors and complement to tag pathogens for destruction. IgG1 and IgG3 bind Fc gamma receptors on phagocytes (like macrophages and neutrophils) with high affinity and efficiently activate the classical complement pathway. This combination makes them particularly potent at opsonizing microbes and driving phagocytosis, which is especially important for clearing infections at mucosal surfaces where pathogens are often encountered by circulating immune components. IgG3 tends to be especially strong in activating complement, and IgG1 also engages Fc receptors effectively, making them the best choices for promoting opsonization and Fc-mediated clearance.

In comparison, IgG2 has weaker Fc receptor engagement and is more associated with responses to polysaccharide antigens, while IgG4 has limited ability to fix complement and lower pro-inflammatory effector signaling. IgA, while crucial at mucosal surfaces in secretions, is not an IgG subclass, and IgM, though good at activating complement early, is not the long-term memory antibody.

So, the IgG1 and IgG3 combination stands out as the most capable of promoting opsonization and Fc receptor–mediated clearance of infections at mucosal sites.