The key difference between IgG IgM IgA IgE and IgD is the type of heavy chain in each antibody. While IgG has a γ type of heavy chain, IgM has a μ type of heavy chain. In contrast, IgA has α type of heavy chain, IgE has ε type of heavy chain, and IgD has δ type of heavy chain.
Antibody production takes place in response to an antigen as a part of the activation of the adaptive immune mechanisms in higher-level animals. The antibody-antigen interaction activates reactions such as agglutination, neutralization, opsonization, complement activation, and B cell activation that take part in facilitating an immune response mechanism against a foreign organism. Antibodies vary in their structural and functional aspects.
CONTENTS
1. Overview and Key Difference
2. What is IgG
3. What is IgM
4. What is IgA
5. What is IgE
6. What is IgD
7. Similarities – IgG IgM IgA IgE and IgD
8. IgG vs IgM vs IgA vs IgE vs IgD in Tabular Form
9. Summary – IgG vs IgM vs IgA vs IgE vs IgD
What is IgG?
Immunoglobulin G or IgG is the most common class of immunoglobulin that is present in large amounts in tissue fluids and blood. It has a serum concentration of over 75%. The molecular weight of IgG is 150,000 D. IgG is a monomer, and the heavy chains of IgG belong to the γ type.
Figure 01: 2D and 3D structures of IgG
There are two antigen-binding sites. The subclasses of IgG include IgG1, IgG2, IgG3, and IgG4. IgG is the only type of immunoglobulin that can cross the placenta. IgG takes part in inducing immune protection since infancy as IgG is also passed from mother to baby during lactation. The key function of IgG is to take part in opsonization and neutralization immune reactions. It also takes part in activating secondary responses during immune reactions.
What is IgM?
Immunoglobulin M or IgM has a unique pentamer structure and thus is the largest type of antibody. Having a molecular weight of about 900,000 D, it accounts for nearly 10% of the total antibody in the serum. The unique pentamer structure facilitates 10 antigen-binding sites. The heavy chains of IgM are composed of the μ type. It also has a characteristic disulfide bond connecting each monomer.
Figure 02: IgM Activation in Bacterial Infections
The main function of IgM is to activate the primary immune response. It is also a good activator of the complement system and takes part in agglutination. Therefore, IgM plays an important role in bacterial infections.
What is IgA?
Immunoglobulin A or IgA is a dimeric antibody that contains a special element called a secretory element. Due to its structural arrangement, the molecular weight is more than that of the antibody classes that have monomeric structures. It is 385,000.00 D. It has 4 antigen-binding sites with heavy chains composed of the α type. It has two subclasses: IgA1 and IgA2.
Figure 03: 3D Structure of IgA
Due to its ability to act as a secretory element, this type of antibody is mainly found in body secretions such as tears, saliva, respiratory and intestinal secretions, and mucosal secretions. In addition, it is also present in colostrum. Around 15% of the total serum antibody is responsible for the class of IgA antibodies. IgA does not take part in activating the complement system; however, its main function is to be expressed in the tissue, preventing the colonization of foreign organisms such as bacteria.
What is IgE?
Immunoglobulin E or IgE is the type of immunoglobulin that is found least in the serum. It accounts for less than 0.01 % of the total serum immunoglobulins. Its structure is monomeric, and the heavy chain is composed of the ε type. Moreover, its molecular weight is around 200,000 D. IgE has two antigen-binding sites similar to that of IgG.
Figure 04: Different Confirmations of IgE
The main function of IgE is to activate allergy reactions in response to conditions like asthma and hay fever. In a broader aspect, IgE activation is observed during type I hypersensitivity reactions. In response to antibody-antigen interaction, secretion of histamines is promoted. It does not have the ability to activate the complement system. It also plays a role in defense against parasitic infections.
What is IgD?
Immunoglobulin D or IgD is also a monomeric immunoglobulin that has only 2 antigen-binding sites. It has the lowest molecular weight, around 185,000 D. IgD accounts for about <0.5% of the total antibody concentration in the serum. The heavy chain is composed of the δ type. The role of IgD is not highly specific, but they mainly take part in activating B cells during the adaptive immune response. They do not activate the complement system. They are unable to cross the placenta.
What are the Similarities Between IgG IgM IgA IgE and IgD?
- All are composed of heavy chains and light chains.
- They are glycoproteins.
- Moreover, all antibody types could facilitate antibody-antigen binding.
- Additionally, antibody–antigen-binding is facilitated due to the presence of antigen-binding sites in all antibodies.
- They are present in the serum.
- All take part in activating adaptive immune responses.
- All antibodies could be detected using immune techniques like Radio Immuno Assay (RIA) or Enzyme-Linked Immunosorbent Assay (ELISA).
- They play an important role in diagnostics and pathology.
What is the Difference Between IgG IgM IgA IgE and IgD?
The key difference between IgG IgM IgA IgE and IgD is the type of heavy chain each antibody type possesses. IgG has the γ type of heavy chain; IgM has the μ type of heavy chain; IgA has α type of heavy chain; IgE has ε type of heavy chain, and IgD has δ type of heavy chain. In addition, their structural arrangements also vary, resulting in varied molecular weights for each type of antibody. Furthermore, the manner in which each antibody acts also varies. While IgG and IgM can activate the complement system, the other types of antibodies are not capable of doing so. Moreover, only IgG can cross the placenta.
The below infographic presents the differences between IgG IgM IgA IgE and IgD in tabular form for side by side comparison.
Summary – IgG vs IgM vs IgA vs IgE vs IgD
Antibodies are derived from B cells, and they play an important role in adaptive immunity. There are five main classes of antibodies that primarily differ based on the type of heavy chain they possess. IgG IgM IgA IgE and IgD have γ, μ, α, ε and δ types of heavy chains, respectively. In addition, they also differ in their structures as IgG, IgE, and IgD attain monomeric structures, IgM attains a pentameric structure, and IgA attains a dimeric structure. The way they activate the immune responses is also different among different classes of antibodies. IgG and IgM are capable of activating the complement system, but the other types are not. So, this is the summary of the difference between IgG IgM IgA IgE and IgD.
Reference:
1. Schroeder, Harry W, and Lisa Cavacini. “Structure and Function of Immunoglobulins.” The Journal of Allergy and Clinical Immunology, U.S. National Library of Medicine, Feb. 2010.
2. Vaillant, Angel A. Justiz. “Immunoglobulin.” StatPearls. U.S. National Library of Medicine, 12 Oct. 2021.
Image Courtesy:
1. “2220 Four Chain Structure of a Generic Antibody-IgG2 Structures” By OpenStax College – Anatomy & Physiology, Connexions Web site. Jun 19, 2013. (CC BY 3.0) via Commons Wikimedia
2. “Immunoglobulin (IgM) antibodies binding to adjacent antigen epitopes on the surface of bacterial cells” By SA1590 – Own work (CC BY-SA 4.0) via Commons Wikimedia
3. “3D structure of secretory IgA2” By McortNGHH – Own work (CC BY-SA 4.0) via Commons Wikimedia
4. “Mode de fonctionnement du récepteur Fc avec une immunoglobuline E” By Ben Mkaddem Sanae, Benhamou Marc, Monteiro Renato C.Understanding Fc Receptor Involvement in Inflammatory Diseases: From Mechanisms to New Therapeutic Tools. Frontiers in Immunology 10,2019, page 811 (CC BY-SA 4.0) via Commons Wikimedia