Antibody Structure and Variable Domains

Steps:

• A typical Y-shaped antibody (IgG) consists of two heavy chains and two light chains linked by disulfide bonds.
• Each light chain has one variable region (VL) at its N-terminus.
• Each heavy chain has one variable region (VH) at its N-terminus.
• Total variable regions: 2 VL + 2 VH = 4, forming the antigen-binding sites.

Why C is correct:

• By definition, immunoglobulins have four variable domains (two VH and two VL) that enable antigen specificity in the Fab regions.

Why the others are wrong:

• A. 2: Counts only the two VL domains, ignoring the VH domains.
• B. 3: No structural basis; antibodies have even symmetry with paired chains.
• D. 6: Overcounts by possibly including constant domains or misapplying chain segments.

Final answer: C