Identify the statement that is NOT correct.

To determine which statement is NOT correct, we examine each option against the established structure of an immunoglobulin (antibody).

1. The heavy and light chains are held together by disulfide bonds.
Antibodies consist of two identical heavy chains and two identical light chains. Each heavy‑light pair is linked by inter‑chain disulfide bonds, which stabilize the overall Y‑shaped structure. This statement is correct.

2. Antigen binding site is located at C‑terminal region of antibody molecules.
The antigen‑binding sites (also called paratopes) are formed by the variable (V) domains of both the heavy and light chains. These V‑domains are positioned at the N‑terminal (the “arms” of the Y) of the Fab fragment, not at the C‑terminal end. The C‑terminal portions contain the constant (C) domains that mediate effector functions. Therefore, this statement is false and is the correct answer to the question.

3. Constant region of heavy and light chains are located at C‑terminus of antibody molecules.
The constant (C) regions indeed lie toward the C‑terminal end of each chain, forming the Fc portion that interacts with immune cells and complement. This statement is accurate.

4. Each antibody has two light and two heavy chains.
The classic immunoglobulin architecture comprises exactly two identical heavy chains and two identical light chains, giving the characteristic Y‑shape. This statement is correct.

Since only statement 2 contradicts the known structural organization of antibodies, it is the statement that is NOT correct.