Antibodies and Immunoglobulin Classes
One-Sentence Definition
Antibodies (immunoglobulins) are B-cell–derived glycoproteins that bind antigens with specificity — IgM, IgG, IgA, IgE, and IgD differ in structure, distribution, and diagnostic/clinical roles in infectious disease.
Simple Explanation
Y-shaped proteins that tag microbes for destruction. Different classes work in blood, mucosa, or allergy — labs measure them for diagnosis.
Detailed Scientific Explanation
| Class | Main site | Microbiology role |
|---|---|---|
| IgM | Blood (pentamer) | First response; acute infection serology |
| IgG | Blood, tissues | Memory, opsonization, complement activation, placental transfer |
| IgA | Mucosa (secretory) | Respiratory/GI/Urogenital defense; breast milk |
| IgE | Mast cells/basophils | Parasites; allergic reactions to drugs/organisms |
| IgD | B-cell surface | Less central clinically |
Functions
- Neutralization (toxins, viruses)
- Opsonization (enhance phagocytosis)
- Complement activation (IgM/IgG)
- ADCC
Serology interpretation (see Serology in Clinical Microbiology)
- IgM suggests recent/acute (with caveats)
- IgG may indicate past infection, immunity, or remote exposure
- Fourfold rise in paired sera = evidence of recent infection
- Vaccine-induced IgG (anti-HBs, anti-tetanus) ≠ always disease
Mechanism
V(D)J recombination → somatic hypermutation → class switching under T-cell help.
Clinical Importance
- RhIG, IVIG, hyperimmune globulins (rabies, HBV PEP)
- Monoclonal antibody therapeutics (RSV, SARS-CoV-2)
Research Importance
- Epitope mapping; AI for Vaccine Design
Diagnostic Relevance
AMR Relevance
- None direct; serology guides when antibiotics are not needed (viral syndromes)
Related MOCs
Active Recall Questions
- Which Ig class crosses the placenta?
- IgM vs IgG in acute vs convalescent serology?
- Secretory IgA — where does it protect?