The region of an antibody that contains the antigen-binding sites is the

• A. Fc region
• B. Fab region
• C. Hinge region
• D. Constant region of the heavy chain

Answer: (B)

Antigen recognition happens at the variable portions of the antibody that come together to form the binding site, and these are contained in the Fab region. The Fab fragment includes the variable domains of both the light and heavy chains, whose variable loops (the complementarity-determining regions) assemble to create the antigen-binding pocket. This binding site is specific to a particular epitope on the antigen and is what drives the immune specificity of the antibody. The Fc region, in contrast, is the constant tail of the antibody that mediates effector functions like binding to Fc receptors and activating complement, not antigen binding. The hinge region is simply a flexible linker between Fab and Fc, providing movement but not contributing binding sites. The constant region of the heavy chain is part of the Fc portion and likewise does not participate in antigen recognition.