Difference Between IgM and IgG

Key Difference – IgM vs IgG
 

Immunoglobulin M (IgM) and Immunoglobulin G (IgG) are antibodies or immunoglobulin (Ig) proteins produced by the immune system to fight against the infections and destroy antigens. IgM is a pentameric molecule that appears in the early stage of infection and it has ten antigen binding sites. IgG is a monomeric molecule that appears at later infection and has two antigen binding sites. This is the key difference between IgM and IgG. The following information regarding IgM and IgG will help you to understand the structural and functional difference between IgM and IgG.

CONTENTS
1. Overview and Key Difference
2. What is an Immunoglobulin (Ig)
3. What is IgM
4. What is IgG
5. Side by side Comparision – IgM vs IgG
6. Summary

What is an Immunoglobulin (Ig)?

Immunoglobulin (Ig), also referred as an antibody,  is a type of protein produced by the white blood cells of the immune system to react against the infections caused by bacteria, viruses, fungi, protozoan, toxin, etc. Ig is a Y-shaped, large glycoprotein molecule consisting of four polypeptides known as heavy and light chains as shown in figure 01. There are two major regions of the polypeptide chain: variable and constant. Amino acid sequences in the variable region of the polypeptides greatly vary among the immunoglobulin isotypes. There are five major isotypes of immunoglobulins: IgA, IgD, IgE, IgG and IgM. Isotypes are categorized according to their structural differences. They do have different functions and antigen responses.

What is IgM?

IgM is the first type of antibody made in the body as the first response to an infection by the immune system. It is the largest antibody found in the body and less abundant (5 to 10%) than the other antibodies. IgMs are produced by plasma cells and are present in blood and lymph fluids. IgM exists as a pentamer consisting of identical heavy and light chains as shown in figure 02. There are ten antigen binding sites for the IgM. However, due to the conformational constraints of the IgM, only five sites are available for antigen binding. IgM is responsible for the early destruction of the antigen and control of infection.

What is IgG?

IgG is another type of antibody produced by white blood cells and are found in all body fluids. It is the most abundant antibody found in the immune system (80%) and the smallest antibody. IgGs are produced at the later stages of the infection and remain in the body to fight against repeated infections. IgG antibodies are able to cross the placenta of a pregnant mother and protect the fetus from infections due to its small size. IgGs exist as monomers with two antigen binding sites in each antibody as shown in figure 02.

What is the difference between IgM and IgG?

IgM vs IgG
IgM is a pentameric molecule that appears in the early stage of infection.	IgG is a monomeric molecule that appears at later stage of infection.
First Appearance in an Organism
First antibody is produced by the virgin plasma cells in the fetus.	This is not the first antibody produced by the virgin plasma cells of the fetus.
Size and Abundance
IgM is the largest antibody but the least abundant antibody in the body.	IgG is the smallest and highly abundant antibody in the body.
Structure
IgM is a pentameter.	IgG is a monomer.
Presence
It is found in the blood and lymph fluid.	It is found in all body fluids.
Antigen Binding Sites
It has 10 or 12 binding sites for antigens.	It has two binding sites for antigens.
Placenta
Since it’s a larger antibody, can’t cross the placenta.	It’s the only antibody type can cross the placenta and build the immunity of the fetus.
Presence in Colostrum
IgM is absent in the colostrum.	IgG is present in the colostrum.
Types
There is only one type of IgMs.	There are four types of IgGs.
Immunology Test
IgM indicates the current infection.	Immunology test indicates the recent or past occurrence of the infection.

Summary –  IgM vs IgG

Both (IgM) and (IgG) are types of immunoglobulin proteins found in the immune system to fight against infections. They are antibodies synthesized by the plasma cells to bind with specific foreign antigens, which enter the body followed by infections. Once these antibodies bind to specific antigens, the immune system can identify the infective cells and destroy the pathogens.

IgM antibodies appear as soon as the body is exposed to the infection while IgG antibodies appear after few days of the infection when the IgM antibodies have disappeared from the body. This is the key difference between IgM and IgG.

 Reference:
1. “IMMUNOGLOBULINS – STRUCTURE AND FUNCTION.” IMMUNOGLOBULINS – STRUCTURE AND FUNCTION. N.p., n.d. Web. 08 Feb. 2017
2. “Textbook of Microbiology & Immunology.” Google Books. N.p., n.d. Web. 08 Feb. 2017

Image Courtesy:
1. “Mono-und-Polymere” By Martin Brändli (brandlee86) – Own work (CC BY-SA 2.5) via Commons Wikimedia
2. “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)