The correct answer is IgG.

Of the five immunoglobulin isotypes, immunoglobulin G (IgG) is most abundant in human serum. The four subclasses, IgG1, IgG2, IgG3, and IgG4.

IgG accounts for about 10–20% of plasma protein.

Each IgG molecule consists of the basic four-chain immunoglobulin structure—two identical H chains and two identical L chains (either kappa or lambda)—and thus carries two identical antigen-binding sites.

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