Immunoglobulin Structure

Immunoglobulins or antibodies are glycoprotein molecules that are produced by plasma cells (white blood cells). They play an important role in the immune response as they specifically recognize and bind to particular antigens, such as viruses or bacteria, thus helping in destroying them. The antibody immune response is extremely specific and complex. The various immunoglobulin types and their subclasses (isotypes) are different from each other based on their structure, biological features, distribution, and target specificity. Immunoglobulin structure and classes knowledge is important for the selection and preparation of antibodies as immunoassays tools and as other detection applications. The immunoglobulin diagram explains its structure.

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The glycoproteins in immunoglobulins are made up of one or more units, each having four polypeptide chains: two identical light chains (L) and two identical heavy chains (H). The amino-terminal ends of the polypeptide chains vary in amino acid composition. They are known as the variable (V) regions and are different from the relatively constant (C) regions. There is one variable domain in every L chain along with one constant domain, CL. The H chains have a variable domain, VH with three constant domains: CH1, CH2,and CH3. Each heavy chain has about twice the number of amino acids and molecular weight (~50,000) than those in each light chain (~25,000). This gives a total immunoglobulin monomer molecular weight of approximately 150,000.

The heavy and the light chains are joined by covalent interchain disulfide bonds and non-covalent interactions. This forms a bilaterally symmetric immunoglobulin structure. The V regions of both H and L chains consist of the antigen-binding sites of the immunoglobulin (Ig) molecules. Each Ig monomer has two antigen-binding sites and is bivalent. The H chains area between the first and second C region domains is the hinge region and it is bound together by disulfide bonds. This flexible hinge region is present in IgG, IgA, and IgD, but not there in IgM or IgE. It is responsible for the varying distance between the two antigen-binding sites.

Classification of Immunoglobulin

Immunoglobulin can be classified into five types: IgG, IgM, IgA, IgD, and IgE. Out of these classes of immunoglobulins, some also include several distinct subclasses. The structural variations of the H chains determine the immunoglobulin types structure and function. Although there are five different types of H chains there are only two basic kinds of L chains: lambda (λ) and kappa (κ) chains. Either of these chains can be associated with any of the H chain classes which further enhance the already enormous diversity of immunoglobulins.

Classification of Immunoglobulin in Tabular Format

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Types of Antibodies

The difference in valency of antibody classes is due to the different numbers of Y-like units (monomers) that combine to form the complete protein. For instance, the functioning IgM antibodies in humans consist of five Y-shaped units (pentamer) with a total of 10 light and heavy chains each along with 10 antigen-binding.

This classification of immunoglobulin can be distinguished based on the heavy chain type found in the molecule. IgMs have my-chains, IgG molecules have heavy chains known as gamma-chains, IgEs have epsilon-chains, IgAs have alpha-chains and IgDs have delta-chains. These differences in heavy chain polypeptides are responsible for the functioning of these different immunoglobulin types at various stages of immune responses. The polypeptide protein sequences are the cause for these differences and they are found mainly in the Fc fragment.