Immunoglobulin Response

In the clinical microbiology laboratory, serological testing plays a vital role in diagnosing and monitoring infectious diseases. A key component of this testing revolves around understanding the body’s antibody response, specifically the roles of Immunoglobulin M (IgM), Immunoglobulin G (IgG), and Immunoglobulin A (IgA). These immunoglobulins, produced in response to infection, each have unique characteristics and functions that provide valuable insights into the stage and nature of an infection, as well as the individual’s immune status. This review will explore the analytic procedures used to detect and interpret these antibodies, highlighting their clinical significance in the diagnosis and management of infectious diseases

The Basics: Immunoglobulins (Antibodies)

• What are they?: Glycoprotein molecules produced by plasma cells (differentiated B lymphocytes) in response to an antigen (foreign substance)
• Function: To recognize, bind, and neutralize/eliminate antigens like bacteria, viruses, toxins, etc
• Structure: Basic Y-shaped structure with:
  • Fab region: Antigen-binding fragment (variable region – determines specificity)
  • Fc region: Constant region – mediates effector functions (e.g., complement activation, binding to phagocytes)
• Classes/Isotypes: IgG, IgM, IgA, IgE, IgD. Each has distinct properties and roles

IgM: The Early Responder

• Characteristics
  • Pentamer: Largest antibody, with 10 antigen-binding sites
  • First antibody produced: in response to a new infection (primary immune response)
  • Good at
    • Agglutination (clumping of antigens, e.g., bacteria)
    • Complement activation (powerful immune cascade leading to pathogen lysis)
  • Stays in the bloodstream: Due to its large size, it doesn’t diffuse well into tissues
• Clinical Significance
  • Indicates acute or recent infection.: A positive IgM result often suggests an active or recent infection
  • Can be falsely positive: Rheumatoid factor (RF) can bind to IgG-Fc, causing false positive IgM results. Also, polyclonal B cell activation
  • Doesn’t cross the placenta: Useful for diagnosing congenital infections in newborns (e.g., congenital rubella - if the baby has IgM antibodies, it means they were infected in utero)
• In the Lab
  • Testing: IgM-specific immunoassays are used (e.g., ELISA, chemiluminescence)
  • Interpretation: A positive IgM result should be interpreted in conjunction with clinical findings and other lab results

IgG: The Long-Term Protector

• Characteristics
  • Monomer: Smaller than IgM
  • Most abundant antibody: in serum
  • Produced later: in the immune response (after IgM) - part of the secondary immune response
  • Longer half-life: than IgM
  • Good at
    • Neutralizing toxins and viruses
    • Opsonization (coating pathogens to enhance phagocytosis)
    • Complement activation (though less efficient than IgM)
    • Antibody-dependent cell-mediated cytotoxicity (ADCC)
  • Crosses the placenta: Provides passive immunity to the fetus/newborn
• Clinical Significance
  • Indicates past infection or vaccination.: A positive IgG result often suggests previous exposure or immunity
  • Can indicate chronic infection: In some chronic infections, IgG levels remain elevated
  • Used to assess immunity: IgG titers (levels) can be measured to determine if a person has protective immunity (e.g., after vaccination)
• In the Lab
  • Testing: IgG-specific immunoassays are used
  • Subclasses: IgG has four subclasses (IgG1, IgG2, IgG3, IgG4), each with slightly different functions. Some assays can differentiate between subclasses
  • Avidity: IgG avidity (strength of binding) increases over time. High avidity IgG suggests a more mature, long-standing infection

IgA: The Mucosal Guardian

• Characteristics
  • Monomer in serum, dimer in secretions.
  • Main antibody found in mucosal secretions: Tears, saliva, breast milk, respiratory secretions, gastrointestinal secretions
  • Secretory component: In secretions, IgA is bound to a secretory component, which protects it from degradation
  • Good at
    • Neutralizing pathogens at mucosal surfaces
    • Preventing pathogen attachment to epithelial cells
    • Immune exclusion (preventing antigens from crossing the mucosal barrier)
• Clinical Significance
  • Mucosal immunity: Important for protection against respiratory and gastrointestinal infections
  • Breast milk: Provides passive immunity to infants
  • Selective IgA deficiency: Most common primary immunodeficiency
• In the Lab
  • Testing: IgA-specific immunoassays are used. Often performed on serum, but can also be performed on other body fluids (e.g., saliva)
  • More challenging: Measuring IgA in secretions can be more challenging due to variability in sample collection and matrix effects

Key Differences Summarized

Feature	IgM	IgG	IgA
Structure	Pentamer	Monomer	Monomer (serum), Dimer (secretions)
Response	Early, primary	Later, secondary	Mucosal immunity
Abundance	Lower in serum	Highest in serum	High in secretions
Function	Agglutination, complement	Neutralization, opsonization, complement, ADCC	Neutralization at mucosal surfaces, immune exclusion
Clinical Use	Acute infection	Past infection, immunity, chronic infection	Mucosal infections, IgA deficiency
Placental Transfer	No	Yes	No

Important Considerations for the Clinical Lab

• Assay Selection: Choose appropriate assays based on the clinical question and the suspected pathogen
• Quality Control: Follow strict QC procedures to ensure accurate and reliable results
• Interference: Be aware of potential interferences (e.g., rheumatoid factor, heterophile antibodies) and use appropriate controls
• Interpretation: Interpret results in the context of the patient’s clinical history, symptoms, and other lab findings
• Communication: Communicate clearly with clinicians about the limitations of serological testing and the appropriate interpretation of results

VII. Example Scenarios

• Patient with suspected acute hepatitis A: Test for IgM anti-HAV
• Patient with history of chickenpox exposure: Test for IgG anti-VZV to determine immunity
• Infant with suspected congenital CMV infection: Test for IgM anti-CMV (in the infant)
• Patient with recurrent respiratory infections: Consider testing for IgA deficiency

Key Terms

• Antigen: A substance (e.g., bacterium, virus, toxin, or other foreign material) that triggers an immune response in the body, specifically stimulating the production of antibodies
• Antibody (Immunoglobulin): A glycoprotein produced by plasma cells in response to an antigen, capable of binding specifically to that antigen. Antibodies mediate the humoral immune response and facilitate the neutralization or elimination of the antigen
• IgM (Immunoglobulin M): The largest antibody isotype, typically the first antibody produced during a primary immune response. Its presence often indicates a recent or acute infection
• IgG (Immunoglobulin G): The most abundant antibody isotype in serum, produced during the later stages of an infection and providing long-term immunity. It can cross the placenta, providing passive immunity to the fetus
• IgA (Immunoglobulin A): The predominant antibody isotype found in mucosal secretions (e.g., saliva, tears, respiratory secretions), providing localized immunity at mucosal surfaces
• Seroconversion: The development of detectable antibodies in serum as a result of infection or immunization. It represents a change in serological status from antibody-negative to antibody-positive
• Titer: A measurement of the concentration of an antibody in serum, typically expressed as the reciprocal of the highest dilution of the serum that still yields a positive result in a serological assay
• Avidity: The overall strength of the binding between an antibody and an antigen, reflecting the combined affinity of all binding sites. Higher avidity antibodies generally indicate a more mature immune response
• Cross-reactivity: The ability of an antibody to bind to an antigen that is structurally similar but not identical to the antigen that originally stimulated its production. This can lead to false-positive results in serological assays
• Immunoassay: A biochemical test that measures the presence or concentration of a substance (analyte) in a biological sample (e.g., serum) using the specific binding between an antibody and an antigen. Common immunoassay formats include ELISA, chemiluminescence, and immunofluorescence assays