Core properties of IgG antibodies: structure, chemistry, and secretion Which statements correctly describe immunoglobulin G (IgG)?

Introduction / Context:
IgG is the most abundant antibody class in human serum and a workhorse in adaptive immunity. Understanding its basic properties is foundational for immunodiagnostics, vaccinology, and therapeutic antibody design.

Given Data / Assumptions:

• IgG is a heterotetramer: two heavy chains and two light chains.
• It is glycosylated, typically at the Fc region (for example, Asn297 in human IgG1).
• It is secreted by plasma cells into extracellular fluids including blood.

Concept / Approach:
The structural arrangement (two heavy plus two light chains) forms two antigen-binding sites. Glycosylation modulates Fc receptor engagement and complement activation. Circulating IgG provides systemic protection via neutralization, opsonization, and complement pathways.

Step-by-Step Solution:
Confirm subunit count: IgG contains four polypeptide chains.Confirm chemistry: presence of carbohydrate moieties defines IgG as a glycoprotein.Confirm localization: secreted into blood and interstitial spaces.Therefore, all statements A–C are correct simultaneously.

Verification / Alternative check:
Serological measurements show IgG predominance in serum; mass spectrometry reveals glycoforms; X-ray structures demonstrate the heterotetrameric architecture.

Why Other Options Are Wrong:
“Membrane lipids without any protein component” is unrelated to antibodies and is incorrect.

Common Pitfalls:
Assuming all glycosylation sites are identical across subclasses; glycosylation patterns and effector functions can vary among IgG subclasses and species.

Final Answer:
All of the above.