constant region

antibody structure

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July 16, 2026

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Antibodies, also known as immunoglobulins (Ig), are Y-shaped proteins produced by B lymphocytes that help the immune system fight foreign substances like bacteria and viruses. Each antibody molecule is composed of four polypeptide chains: two identical heavy (H) chains and two identical light (L) chains.12

The "arms" of the Y-shaped antibody contain antigen-binding sites, formed by portions of both heavy and light chains. These sites are unique and recognize specific parts of antigens, called epitopes.1

The antibody molecule also has constant (C) and variable (V) regions within its chains. The variable regions determine the specific shape of the antigen-binding site, allowing for diverse recognition of foreign substances. The constant regions, located in the "tail" of the Y, are responsible for the antibody's biological functions after it binds to an antigen.1

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immune system
Basic structure of the immunoglobulin molecule
…composed of two regions, called constant (C) and variable (V). These regions are distinguished on the basis of amino acid similarity—that is, constant regions have essentially the same amino acid sequence in all antibody molecules of the same class (IgG, IgM, IgA, IgD, or IgE), but the amino acid sequences…
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…composed of two regions, called constant (C) and variable (V). These regions are distinguished on the basis of amino acid similarity—that is, constant regions have essentially the same amino acid sequence in all antibody molecules of the same class (IgG, IgM, IgA, IgD, or IgE), but the amino acid sequences…
Read More