Which immunoglobulin is primarily responsible for systemic immune response in serum?

• A. IgA
• B. IgG
• C. IgM
• D. IgE

Answer: (B)

IgG is the main antibody in the bloodstream and provides the bulk of systemic (serum) immune protection. It circulates at high levels, has high affinity due to memory responses, and handles many systemic tasks such as opsonizing bacteria, neutralizing toxins, and activating the classical complement pathway. This makes it the key defender in serum over the long term.

In contrast, IgM is the first antibody produced during a primary response and is a large pentamer that is very effective at activating complement but is less abundant in steady-state serum. IgA is primarily a mucosal antibody found in secretions like saliva and mucus, offering local protection at mucosal surfaces. IgE plays a role in allergic reactions and defense against parasitic infections.