## Immunoglobulin M (IgM) — Primary Response Antibody ### Structure and Molecular Weight **Key Point:** IgM is a pentameric immunoglobulin with a molecular weight of approximately **900 kDa**, making it the largest antibody in the human immune system. ### Timeline in Primary Immune Response **High-Yield:** IgM appears **first** during primary immune response (within 1–2 weeks of antigen exposure), followed by a switch to IgG (which becomes dominant in secondary response). ### Structural Features | Feature | IgM | IgG | | --- | --- | --- | | **Molecular Weight** | 900 kDa (pentamer) | 150 kDa (monomer) | | **Structure** | 5 basic units + J chain | 4 basic units | | **Valency** | 10 antigen-binding sites | 2 antigen-binding sites | | **Half-life** | 5 days | 21 days | | **Response Timing** | Primary (first) | Primary (later) → Secondary (dominant) | | **Complement Activation** | Excellent (1 IgM molecule sufficient) | Good (requires 2 IgG molecules) | ### Clinical Significance **Clinical Pearl:** Detection of IgM antibodies in serum indicates **recent or acute infection**, whereas IgG indicates past infection or immunity. **Mnemonic:** **"M for Many and Mighty"** — IgM is the first (Many early responders) and largest (Mighty in size) antibody. ### Why IgM is Ideal for Early Response 1. Pentameric structure provides 10 binding sites → high avidity 2. Excellent at agglutination and complement activation 3. Cannot cross placenta (large size) → protects fetus from maternal IgM 4. Confined to intravascular space due to size [cite:Robbins 10e Ch 6]
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