The Fab portion of an antibody contains the hypervariable region.

• A. Binds T-cell receptor
• B. Consists of two light chains only
• C. Consists of two heavy chains only
• D. Contains the hypervariable region

Answer: (D)

The part of an antibody that binds antigen is the Fab fragment, and the sites that determine exactly what antigen is recognized lie in the variable regions of both the heavy and light chains within this fragment. The hypervariable loops in these variable domains—the complementarity-determining regions—form the antigen-contacting surface, giving each antibody its specific binding. So the hypervariable region is contained in the Fab portion.

In contrast, the Fc portion is the constant region of the antibody that mediates effector functions like binding to Fc receptors and activating complement; it does not participate in antigen recognition. The Fab is made up of one light chain and one heavy chain (including the VH and VL variable regions), not two light chains or two heavy chains.