The typical structure of an immunoglobulin includes two light chains and two heavy chains.

• A. 2L and 2H chains
• B. 4L and 2H chains
• C. 4L and 4H chains
• D. 2L and 4H chains

Answer: (A)

The structure of a typical immunoglobulin is a heterotetramer built from two identical light chains and two identical heavy chains, held together by disulfide bonds to form a Y-shaped molecule. Each arm of the Y pairs one light chain with one heavy chain, creating two antigen-binding sites. Because there are two arms, the molecule contains two light chains and two heavy chains in total. The light chains come in two possible types (kappa or lambda), but this does not change the count of chains. The other options suggesting four light chains, four heavy chains, or a mix that doesn’t pair into two arms don’t reflect how antibodies are assembled.