From the Guidelines
IgM plays a crucial role in the initial immune response, distinct from other immunoglobulins, and is the primary indicator of acute hepatitis B infection, as evidenced by the presence of IgM anti-HBc 1.
Key Characteristics of IgM
- IgM is the first antibody produced during an initial immune response, appearing before other antibody classes
- It exists primarily as a pentamer, giving it ten antigen-binding sites which makes it highly effective at agglutinating pathogens and activating complement
- IgM cannot cross the placenta due to its large size, but it is found in secretions and blood
Comparison with Other Immunoglobulins
- IgG is the most abundant antibody in serum, provides long-term immunity, crosses the placenta, and has four subclasses with varying functions
- IgA protects mucosal surfaces in secretions
- IgE mediates allergic reactions and defends against parasites
- IgD functions primarily as an antigen receptor on B cells
Clinical Significance of IgM in Hepatitis B
- IgM anti-HBc is a reliable marker of acute HBV infection, persisting for up to 6 months after infection 1
- The presence of IgM anti-HBc indicates recent exposure to HBV and is used to distinguish acute from chronic infection 1
- The use of IgM anti-HBc testing is recommended for diagnosis of acute hepatitis B, particularly in individuals with clinical evidence of acute hepatitis or an epidemiologic link to a person with HBV infection 1.
From the Research
Role of IgM in Immune Response
- IgM provides the first line of defense during microbial infections, prior to the generation of adaptive, high-affinity IgG responses 2
- IgM plays an important role in immune regulation and immunological tolerance 3
- IgM can elicit its function through the IgM Fc receptor (FcμR), in addition to binding to antigen and activating the complement system 3
Comparison with Other Immunoglobulins
- IgM is the first antibody isotype to appear during evolution, ontogeny, and immune responses 3
- IgM responses are commonly used as a measure of exposure to infectious diseases, whereas IgG responses are important for long-lived immunity and immunological memory 2
- Selective IgM deficiency can lead to impaired immune responses, including increased susceptibility to infections and autoimmune diseases 4
Diagnostic and Clinical Significance
- Detection of IgM anti-HAV is a highly specific and sensitive method for the diagnosis of type A hepatitis 5
- IgM anti-HBc is a useful marker for the diagnosis of acute hepatitis B 6
- Rapid, fully automated immunoassays can be used to detect IgM antibodies and differentiate between acute viral hepatitis infections 6