What Are the Key Features of 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.
FAQs on Immunoglobulin Structure: Complete Guide for Students
1. What is the basic structure of an immunoglobulin molecule?
An immunoglobulin molecule has a characteristic Y-shape. It is built from four polypeptide chains: two identical heavy (H) chains and two identical light (L) chains. These chains are linked together by strong chemical bonds called disulfide bonds. Each chain has a constant region, which is similar across antibodies of the same class, and a variable region, which is unique and forms the site that binds to germs.
2. What are the five main classes of immunoglobulins found in the body?
There are five major classes, or isotypes, of immunoglobulins, each with a slightly different structure and function. They are:
- IgG: The most abundant antibody in the blood, it can cross the placenta to protect a fetus.
- IgA: Found in secretions like saliva, tears, and breast milk, it protects body surfaces.
- IgM: The first antibody produced in an immune response, it is very effective at clumping pathogens together.
- IgE: Involved in allergic reactions and defence against parasites.
- IgD: Found on the surface of B-cells, it helps to activate them.
3. What is the specific role of the variable and constant regions in an antibody?
The two regions of an antibody have very different jobs. The variable region is at the tip of the 'Y' arms and acts like a specific key. Its unique shape allows it to recognise and bind to one specific part of a pathogen, called an antigen. The constant region forms the base of the 'Y' and is the same for all antibodies in a class. Its job is to signal to other parts of the immune system, like killer cells, to destroy the pathogen once the variable region has locked on.
4. How does the structure of IgM differ from IgG, and why is this important for its function?
While both have the basic Y-shape, their overall assembly is different. IgG exists as a single Y-shaped molecule (a monomer). In contrast, IgM is a large molecule formed by five Y-shaped units joined together (a pentamer). This pentamer structure gives IgM ten antigen-binding sites, making it extremely effective at grabbing and clumping multiple pathogens together right at the start of an infection, which is its primary role.
5. Why is the 'hinge region' in an immunoglobulin's structure so important?
The hinge region is a flexible area that connects the arms of the 'Y' to its stem. This flexibility is crucial because it allows the two arms to move independently. They can open and close, making it easier for the antibody to bind to two antigen sites on a pathogen's surface, even if those sites are spaced at awkward or different distances apart. This greatly improves its ability to latch onto and neutralise threats.
6. If all antibodies have a similar Y-shape, how can our body make millions of different ones to fight so many unique germs?
The key to this incredible diversity lies in the variable regions. While the basic Y-shaped frame (constant region) is similar, the genetic information for the variable regions is shuffled and rearranged in each B-cell as it develops. This process, called V(D)J recombination, creates millions of unique combinations, resulting in an almost limitless variety of antigen-binding sites. This means our body has a pre-existing antibody ready to recognise almost any new germ it encounters.
