What is the Difference Between IgG IgM IgA IgE and IgD

The main difference between IgG IgM IgA IgE and IgD is their structure and function. IgG has the highest opsonization and neutralization activities; IgM is the first antibody transiently increased upon antigen invasion; IgA is expressed in mucosal tissues; IgE is involved in allergy whereas IgD functions as an antigen receptor on activated B cells. Furthermore, IgG, IgE, and IgD remain as monomer antigens, IgA remains either as a dimer or trimer while IgM remains as a pentamer. Moreover, IgA has two subclasses while IgG has four subclasses. 

IgG, IgM, IgA, IgE, and IgD are the five classes of immunoglobulins in placental mammals. They are also known as antibody isotypes. 

Key Areas Covered 

1. What is IgG
     – Definition, Structure, Role
2. What is IgM
     – Definition, Structure, Role
3. What is IgA
     – Definition, Structure, Role
4. What is IgE
     – Definition, Structure, Role
5. What is IgD
     – Definition, Structure, Role
6. What are the Similarities Between IgG IgM IgA IgE and IgD
     – Outline of Common Features
7. What is the Difference Between IgG IgM IgA IgE and IgD
     – Comparison of Key Differences

Key Terms 

IgA, IgD, IgE, IgG, IgM, Humoral Immunity, Immunoglobulin

What is IgG 

IgG is the predominant form of immunoglobulins (75% in the serum) with the longest serum half-life. Generally, the main function of IgG is to provide the main humoral immune response against pathogens that invade the body. It also plays a key role in the secondary immune response.

Figure 1: Primary and Secondary Immune Response

Usually, the binding of IgG to pathogens causes immobilization and agglutination of pathogens through opsonization. Also, it activates the classical pathway of the complement system while neutralizing toxins. In addition, IgG plays a critical role in antibody-dependent cell-mediated cytotoxicity.  IgG is the only form of antibody that is capable of crossing the placenta in order to provide passive immunity to the fetus. IgG has four subclasses. They are IgG1, IgG2, IgG3, and IgG4. 

What is IgM 

IgM is the first form of immunoglobulins expressed during the development of B cells in the primary immune response. Moreover, it is the largest form of antibody and occurs as a pentamer. Generally, IgM binds to the complement component C1, activating the classical pathway which leads to opsonization.

Figure 2: IgM Structure

However, due to the larger size, IgM tends to stay inside blood vessels. Also, it has a lower affinity towards antigens in comparison to IgG. However, IgM has 10 antigen-binding sites due to its pentameric structure. Therefore, its avidity or the overall binding strength is higher than that of the IgG. 

What is IgA 

IgA is the form of immunoglobulins which functions in the mucous membranes. Generally, it occurs in mucous secretions including tears, sweat, saliva, colostrum, and the secretions of the gastrointestinal, respiratory, and genitourinary tracts. It accounts for 15% of the serum antibodies. In addition, IgA occurs either in monomeric or dimeric forms.

Figure 3: IgA Structure

Generally, the dimeric form of IgA is the most prominent and it is a form of secretory IgA. Furthermore, IgA initiates inflammatory responses by interacting with immune effector cells. The two forms of IgA are IgA1 and IgA2 while IgA1 is the predominant form in the serum. 

What is IgE 

IgE is the form of immunoglobulin with the lowest serum concentration as well as the shortest half-life in the serum. However, it is a very potent antibody associated with allergic reactions and hypersensitivity. It also responds to parasitic worm infections.

Figure 4: IgE Structure

Generally, IgE has a higher affinity towards the FcεRI receptor on mast cells, eosinophils, basophils, and Langerhans cells. Therefore, the binding of IgE to allergens triggers the release of histamine from mast cells and basophils, involving in allergy. 

What is IgD 

IgD is another type of immunoglobulins with lower serum concentration. It has a shorter half-life in the serum due to the presence of a hinge region, which undergoes proteolysis. Generally, the main function of IgD is to serve as an antigen receptor on naive B cells.

Figure 5: Five Classes of Immunoglobulins

Although its function is unknown, IgD activates mast cells and basophils to produce antimicrobial factors. 

Similarities Between IgG IgM IgA IgE and IgD 

• IgG, IgM, IgA, IgE, and IgD are the five classes of immunoglobulins classified depending on the presence of five types of Fc regions. 
• They are large, Y-shaped, glycoprotein molecules. 
• Activated plasma cells synthesize these antibodies. 
• Generally, their function is to bind to a specific epitope of a pathogenic antigen. For that, they contain compatible regions called paratopes.  
• They are the components of the humoral immune system. 

Difference Between IgG IgM IgA IgE and IgD 

Definition 

IgG refers to a class of immunoglobulins including the most common antibodies circulating in the blood, facilitating phagocytic destruction of microorganisms while IgM refers to the class of immunoglobulins of higher molecular weight including primary antibodies released into the bloodstream early in the immune response. IgA refers to the class of antibody, playing a crucial role in the immune function of mucous membranes. On the other hand, IgE refers to the class of immunoglobulins produced in response to an allergy. However, IgD refers to a class of immunoglobulins expressed in the plasma membranes of immature B lymphocytes.  

Structure 

IgG, IgE, and IgD remain as monomer antigens, and IgA remains either as a monomer or dimer while IgM remains as a pentamer.  

Subclasses 

IgA has two subclasses and IgG has four subclasses while the rest of the antibodies have a single class. 

Function 

Moreover, IgG has the highest opsonization and neutralization activities, IgM is the first antibody transiently increased upon antigen invasion, IgA is expressed in mucosal tissues, preventing colonization, IgE is involved in allergy while IgD functions as an antigen receptor on activated B cells.  

Fc Receptor 

IgG interacts with FcγR I, II, and III, IgM does not interact with Fc receptors, IgA interacts with FcαR, IgE interacts with FcεR I and II while IgD interacts with FcδR.  

Serum Concentration 

The serum concentration of IgG is 75%; IgM is 10%; IgA is 15%; IgE is <0.01% while IgD is <0.5%. 

Conclusion 

IgG is a monomer responsible for the destruction of pathogens through phagocytosis. On the other hand, IgM is the heaviest immunoglobulin type, occurring as pentamers. Also, they are the primary antibodies produced in response to an immune response. IgA is either monomer, dimer or trimer, occurring in mucous membranes. However, IgE is a monomer produced in response to an allergy. Moreover, IgD is also a monomer, functioning as an antigen receptor on activated B cells. Therefore, the main difference between IgG, IgM, IgA, IgE, and IgD is their structure and function. 

References:

1. Schroeder, Harry W Jr, and Lisa Cavacini. “Structure and function of immunoglobulins.” The Journal of allergy and clinical immunology vol. 125,2 Suppl 2 (2010): S41-52. doi:10.1016/j.jaci.2009.09.046.

Image Courtesy:

1. “IgM and IgG” By Jfdwolff at English Wikipedia – Transferred from en.wikipedia to Commons by Shashenka using CommonsHelper. (Public Domain) via Commons Wikimedia  
2. “IgM white background” By The original uploader was TimVickers at English Wikipedia. – Transferred from en.wikipedia to Commons. (CC BY-SA 3.0) via Commons Wikimedia   
3. “Dimeric IgA schematic 01” By McortNGHH – Own work (CC BY-SA 4.0) via Commons Wikimedia   
4. “IgE” By SariSabban (CC BY-SA 3.0) via Commons Wikimedia  
5. “2221 Five Classes of Antibodies new” By OpenStax College – Anatomy & Physiology, Connexions Web site. (CC BY 3.0) via Commons Wikimedia   

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