Anatomy of an antibody – where antigen binds: In a complete immunoglobulin molecule, the antigen-binding sites are located specifically within which structural regions?

Introduction / Context:
Antibodies bind antigens through highly variable regions formed by both heavy and light chains. Knowing exactly where binding occurs is essential for understanding specificity, affinity maturation, monoclonal antibody engineering, and diagnostic assay design.

Given Data / Assumptions:

• An antibody has two antigen-binding arms (Fab) and one Fc stalk (effector region).
• Complementarity-determining regions (CDRs) lie within variable domains of heavy (VH) and light (VL) chains.
• Fc mediates effector functions rather than antigen recognition.

Concept / Approach:
Each Fab contains paired VH and VL domains. Six CDR loops (three from VH, three from VL) create the paratope that contacts the antigen’s epitope. The Fc portion is composed of constant domains of heavy chains and does not contribute to antigen specificity; instead, it binds Fc receptors and complement once antigen is bound.

Step-by-Step Solution:
Identify structural regions: Fab vs Fc. Locate CDRs within VH/VL of Fab arms. Conclude that antigen binding occurs in Fab variable domains of both chains. Select the option naming Fab regions.

Verification / Alternative check:
Papain digestion yields two Fab fragments that retain antigen binding and one Fc fragment without binding activity, confirming location.

Why Other Options Are Wrong:
Light or heavy constant regions alone do not bind antigen; Fc and hinge regions are not responsible for specificity.

Common Pitfalls:
Confusing Fc-mediated effector functions with antigen recognition; they are separable structurally and functionally.

Final Answer:
Fab regions (variable domains of both heavy and light chains).