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Immunoglobulin Structure

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What is the 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

Class

Molecular weight

H-chain type (MW)

Serum concentration

Total immunoglobulin %

Glycosylation (by weight)

Distribution

Function

IgG

150,000

gamma (53,000)

10 to 16 mg/mL

75%

3%

Intra- and extravascular

Secondary response

IgM

900,000

mu (65,000)

0.5 to 2 mg/mL

10%

12%

Mostly intravascular

Primary response

IgA

320,000 (secretory)

alpha (55,000)

1 to 4 mg/mL

15%

10%

Intravascular and secretions

Protects the mucus membranes

IgD

180,000

delta (70,000)

0 to 0.4 mg/mL

0.2%

13%

lymphocyte surface

Unknown

IgE

200,000

epsilon (73,000)

10 to 400 ng/mL

0.002%

12%

Basophils and mast cells in saliva and nasal secretions

Protection against parasites


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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.

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FAQs on Immunoglobulin Structure

1. What is the IgG class of immunoglobulin and what is its function?

IgG is one of the immunoglobulin types that is present in large amounts in tissue fluids and blood. Each IgG antibody structure consists of the basic four-chain model of immunoglobulin. There are two identical L chains (either kappa or lambda) and two identical H chains. There are two identical antigen-binding sites in IgG. Based on minor differences in the H chains, IgG can be divided into four subclasses with distinct biological properties. IgG is the only immunoglobulin type that can cross the placenta. This helps in providing some degree of immunity to the developing fetus. IgG molecules also get secreted into the mother’s milk. When the infant ingests them, they are transported into the blood, where they generate immunity.

2. What is IgA and where is it found?

IgA is the antibody found in different body secretions such as saliva, tears, intestinal and respiratory secretions, and colostrum which is the first milk produced by lactating mothers after giving birth. IgA is produced by B cells in the mucous membranes of the body. IgA structure includes molecules that bind together and combine with a special protein. This enables the newly formed IgA molecule to be secreted across epithelial cells lining the various organs and ducts. While IgG is considered the most common class of immunoglobulin, IgA is the antibody class that is synthesized maximum daily by the body. Because IgA is not as stable as IgG, it is present in the body in lower amounts at any given time.

3. Give the IgE structure?

Small proportions of B cells constitute IgE. It is present in low concentrations in the blood. There is one four-chain unit in each molecule of IgE and it has two antigen-binding sites similar to the IgG molecule. IgE has the property of binding to the surface of basophils and mast cells since each of its H chains has an extra constant domain CH4.


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