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IgM vs. IgG: What's the Difference?

Edited by Janet White || By Harlon Moss || Updated on October 13, 2023
IgM and IgG are types of antibodies; IgM is often the first to respond to infections, while IgG provides longer-term immunity and can cross the placenta.

Key Differences

IgM, or Immunoglobulin M, is pivotal as it is frequently the initial antibody to be produced in response to an antigen and signals the body's immune system to respond. It's commonly linked with primary immune responses and is instrumental in the agglutination of pathogens. Conversely, IgG, or Immunoglobulin G, is the most abundant antibody in blood and bodily fluids, defending the body against bacterial and viral infections by enhancing phagocytosis and preventing toxins and viruses from attaching to cells.
Notably, IgM is the largest immunoglobulin, and it operates efficiently in the control of blood infections. It demonstrates a pentameric structure, which enables it to bind to several antigens, and it exhibits a prominent role in the primary immune response. In contrast, IgG is smaller and showcases a monomeric form in circulation, making it highly versatile and allowing it to distribute widely throughout the body, including across the placenta to the fetus, granting passive immunity.
The generation of these antibodies provides distinct functionalities within the immune system. When a foreign invader enters the body, IgM is often the first to respond, providing an early barrier and activating complement pathways. IgG, meanwhile, plays a role in the secondary immune response, conferring longer-term immunity and having a higher affinity for antigens compared to other antibodies, making it a crucial component in long-term protection and immunological memory.
In the context of diagnostics, the presence of IgM and IgG antibodies can serve as markers for various stages of infection or immunization. Elevated IgM levels may indicate a current or recent infection due to its early response characteristics. In comparison, the detection of IgG antibodies often indicates past exposure to an infection or successful vaccination, given that it may persist for an extended duration in the blood following exposure to an antigen.
From a clinical perspective, IgM and IgG antibody testing is beneficial for determining the stage and history of an infection in an individual. For diseases like COVID-19, for example, serological testing for IgM and IgG antibodies has been implemented to infer recent exposure or past infection respectively, providing vital information for healthcare providers and epidemiological tracking.

Comparison Chart

Role in Immune Response

First antibody in response, primary defense
Provides long-term immunity, secondary defense



Presence Indicates

Current or recent infection
Past infection or immunization

Duration of Presence

Short-lived in circulation
Can persist long-term in the body

Ability to Cross Placenta

Does not cross the placenta
Can cross the placenta, providing passive immunity to fetus

IgM and IgG Definitions


IgM typically indicates a recent or current infection when detected in blood serology.
Elevated IgM levels suggested the patient was recently exposed to the virus.


IgG is an antibody that confers long-term immunity and protection against reinfections.
After recovery, IgG antibodies continued to provide immunity against the virus.


IgM is an antibody that provides the first line of defense during microbial infections.
Upon infection, the body rapidly produces IgM to initially combat the invading pathogen.


IgG plays a crucial role in immune memory, recognizing and rapidly responding to previously encountered antigens.
The presence of IgG enabled a swift and targeted response to the reintroduced virus.


This antibody, IgM, prominently features in the primary immune response to an antigen.
The detection of IgM indicated an early stage of infection in the patient.


Immunoglobulin G (IgG) can cross the placenta, offering passive immunity to the fetus.
The mother’s IgG antibodies provided initial immune protection to the newborn.


IgM plays a critical role in agglutinating pathogens and activating the complement system.
The IgM antibodies quickly agglutinated the bacteria, preventing it from spreading.


IgG has a monomeric structure and is the most prevalent antibody in blood and extracellular fluid.
The serum contained a high concentration of IgG, indicating a mature immune response.


IgM is the largest immunoglobulin, often exhibiting a pentameric structure.
The substantial size of IgM allows it to effectively bind to and neutralize antigens.


This antibody, IgG, participates in secondary immune responses, recognizing previously encountered pathogens.
The IgG antibodies effectively identified and neutralized the familiar bacterial strain.


One of the five major classes of immunoglobulins; involved in fighting blood infections and in triggering production of immunoglobulin G


To ignore or snub (someone).


A snub or rebuff.


To ignore deliberately.


One of the five major classes of immunoglobulins; the main antibody defense against bacteria


What does IgG stand for?

Immunoglobulin G.

Which antibody offers long-term protection?


Which antibody typically responds first to an infection?


Does IgG play a role in immune memory?

Yes, IgG plays a vital role in immune memory.

Can IgM indicate recent infection?

Yes, elevated IgM often indicates recent infection.

What does IgM stand for?

Immunoglobulin M.

Is IgM generally indicative of chronic or acute infections?

IgM is generally indicative of acute or recent infections.

What is the significance of IgG in vaccinations?

IgG plays a critical role in providing immunity post-vaccination by recognizing and fighting the vaccinated pathogen.

Which immunoglobulin can activate the complement pathway?

Both IgM and IgG can activate the complement pathway, but IgM is particularly efficient at it.

Can both IgM and IgG be used in serological testing?

Yes, both IgM and IgG can be used in serological testing to determine past and recent infections.

What kind of structure does IgM have?

IgM typically has a pentameric structure.

Can the detection of IgG indicate past infection or immunization?

Yes, the detection of IgG can indicate past infection or successful immunization.

What role does IgM play in the immune response?

IgM provides the initial defense, agglutinating pathogens, and activating the complement system.

Do IgG antibodies have a high affinity for antigens?

Yes, IgG antibodies typically have a higher affinity for antigens compared to other antibody classes.

Can IgG cross the placenta?

Yes, IgG can cross the placenta.

Is IgG involved in primary or secondary immune responses?

IgG is involved in secondary immune responses.

Which antibody is more abundant in the bloodstream?

IgG is more abundant in the bloodstream.

Can IgG antibodies recognize and bind to specific antigens?

Yes, IgG can specifically recognize and bind to antigens from previously encountered pathogens.

What is the structural difference between IgM and IgG?

IgM often exists in a pentameric form, while IgG is monomeric in circulation.

How does IgG protect against pathogens?

IgG neutralizes pathogens, enhances phagocytosis, and prevents pathogens from adhering to host cells.
About Author
Written by
Harlon Moss
Harlon is a seasoned quality moderator and accomplished content writer for Difference Wiki. An alumnus of the prestigious University of California, he earned his degree in Computer Science. Leveraging his academic background, Harlon brings a meticulous and informed perspective to his work, ensuring content accuracy and excellence.
Edited by
Janet White
Janet White has been an esteemed writer and blogger for Difference Wiki. Holding a Master's degree in Science and Medical Journalism from the prestigious Boston University, she has consistently demonstrated her expertise and passion for her field. When she's not immersed in her work, Janet relishes her time exercising, delving into a good book, and cherishing moments with friends and family.

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