Which region of the immunoglobulin (antibody) is responsible for binding to antigens?

Variable region of both the heavy and light chain

The correct answer is Option (4) -Variable region of both the heavy and light chain.

Antibodies, also known as immunoglobulins, are protein molecules produced by B-cells as part of the immune response. Each antibody consists of several polypeptide chains. The structure of an antibody includes two types of chains: light chains and heavy chains.

Antibodies have four polypeptide chains in total. There are two small chains called light chains. The small chains are referred to as light chains.  Antibodies have two longer chains called heavy chains. The longer chains are referred to as heavy chains.  The antibody structure is represented as H₂L₂, indicating two heavy chains and two light chains.

The antigen binding site of an immunoglobulin (antibody) is a specific region that is responsible for recognizing and binding to antigens. It is formed by the variable regions of both the heavy and light chains of the antibody molecule.

Immunoglobulins are proteins produced by B-lymphocytes as a part of the adaptive immune response. They play a critical role in identifying and neutralizing foreign substances, known as antigens, such as bacteria, viruses, and other pathogens.

The heavy chain consists of a variable (VH) region and a constant (CH) region, while the light chain consists of a variable (VL) region and a constant (CL) region. The VH and VL regions come together to form the antigen binding site.

In summary, the antigen binding site of an antibody is located in the variable regions of both the heavy and light chains. It is formed by the CDRs within these regions, which provide the specificity for antigen recognition and binding.