A Fab fragment consists of which combination of chains?

• A. two H chains.
• B. two L chains.
• C. one L chain and one-half of an H chain.
• D. one L chain and an entire H chain.

Answer: (C)

The Fab fragment is the antigen-binding portion of an antibody, made from one light chain paired with the portion of one heavy chain that contains the variable region plus the first constant domain. In other words, it includes one light chain and half of a heavy chain (the heavy-chain fragment that carries VH and CH1). This arrangement forms a single antigen-binding site at the interface of the light and heavy chain variable regions.

Proteolytic cleavage of antibodies (for example with papain) yields two Fab fragments and one Fc fragment. Each Fab contains one light chain and the corresponding heavy-chain fragment, while the Fc is the remaining constant region of the two heavy chains. That’s why the correct description is one light chain together with half of a heavy chain.