Overview
IgG is a monomeric immunoglobulin with two antigen-binding sites that typically requires 37°C incubation and antihuman globulin (AHG) reagent for laboratory detection, and is the only immunoglobulin class transported across the placenta.
Classification
| Feature | IgG |
|---|---|
| Structure | Monomer |
| Antigen-binding sites | 2 |
| Optimal thermal phase | 37°C / AHG |
| Placental transfer | Yes |
Morphologic Features
Not a cell-morphology topic. IgG is characterized by immunoglobulin structure and serologic behavior, not microscopy.
Laboratory Characteristics
- Detected via indirect antiglobulin test (37°C incubation + AHG reagent)
- Usually non-reactive at immediate spin/room temperature
- Predominant class in secondary (anamnestic) immune responses
- Subclass (IgG1–IgG4) can influence complement-fixing and placental-transfer behavior
Reference Intervals
IgG antibody detection is categorical (reactive by phase), not a numeric reference interval. Titer methods and critical values for HDFN-associated antibodies are institution- and reference-laboratory-specific, verify SOP.
Clinical and Laboratory Significance
IgG-behaving antibodies (37°C/AHG-reactive) account for most clinically significant unexpected red-cell antibodies and for HDFN risk via placental transfer.
Differential Considerations
AHG-phase reactivity is consistent with IgG but is not itself a specificity result, full panel identification distinguishes which specific antibody is present.
Comparison Tables
IgG vs IgM quick comparison
| Feature | IgG | IgM (see IgM entry) |
|---|---|---|
| Structure | Monomer | Pentamer |
| Direct saline agglutination | Usually no | Often yes |
| Placental transfer | Yes | No |
| Typical detection phase | 37°C / AHG | Immediate spin / RT |
Classification Frameworks
Not applicable, immunoglobulin class is an immunology concept, not a WHO disease classification.
Laboratory Notes
- AHG-only reactivity: think IgG first, then confirm specificity
- Do not assume significance from phase reactivity alone
- Titer/monitoring protocols for HDFN-associated IgG antibodies are institution-specific
References
Authoritative textbooks, guidelines, and reviews supporting this reference entry. Verify reference intervals, critical limits, and reflex criteria against institutional protocols and current guideline editions.
Textbooks
- Cohn CS, Delaney M, Johnson ST, Katz LM, eds. Technical Manual. 21st ed. AABB; 2023.
- Harmening DM. Modern Blood Banking & Transfusion Practices. 7th ed. F.A. Davis; 2019.
- McPherson RA, Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Elsevier; 2021.
CLSI and Professional Guidelines
- AABB. Standards for Blood Banks and Transfusion Services (verify current edition and institutional adoption).
- Clinical and Laboratory Standards Institute. Related immunohematology and specimen identity standards (verify current documents and institutional adoption).
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