Where are the antigen-binding sites located on an antibody molecule?

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Diagram of an antibody molecule showing two identical light chains and two identical heavy chains joined by disulfide bonds in a Y-shaped structure, with labeled regions for antigen binding and effector function. — antibody The four-chain structure of an antibody, or immunoglobulin, molecule. The basic unit is composed of two identical light (L) chains and two identical heavy (H) chains, which are held together by disulfide bonds to form a flexible Y shape. Each chain is composed of a variable (V) region and a constant (C) region.

The antigen-binding sites, also known as antibody-combining sites, are located at the tip of each arm of the Y-shaped antibody molecule.1

Each site is formed by a portion of both the heavy and light polypeptide chains. Specifically, these sites are determined by the variable (V) regions of these chains, which contain unique amino acid sequences that create a specific shape capable of recognizing and binding to a corresponding epitope on an antigen. Because the two arms of the Y-shaped molecule are identical, every immunoglobulin molecule possesses at least two identical antigen-binding sites.1