## Immunoglobulin Classes in Primary vs Secondary Response **Key Point:** IgM is the first antibody produced during the primary immune response, appearing within days of antigen exposure. It is pentameric and highly efficient at complement activation despite its low affinity. **High-Yield:** The temporal sequence of antibody appearance is crucial: 1. IgM appears first (primary response) 2. IgG appears later and becomes dominant (secondary response) 3. IgA is primarily mucosal 4. IgE is rare, involved in allergic/parasitic responses ## Structural and Functional Characteristics | Feature | IgM | IgG | IgA | IgE | |---------|-----|-----|-----|-----| | **Molecular weight** | 900 kDa (pentamer) | 150 kDa | 160 kDa (dimer) | 188 kDa | | **Primary location** | Serum, lymph | Serum, tissues | Mucosal secretions | Mast cells, basophils | | **Complement activation** | Excellent (one IgM molecule) | Good (requires 2 IgG) | Poor | None | | **Crosses placenta** | No | Yes | No | No | | **Half-life** | 5 days | 21 days | 6 days | 2 days | | **Appears in** | Primary response | Secondary response | Mucosal immunity | Allergic/parasitic | **Clinical Pearl:** Detection of IgM antibodies against an antigen indicates acute or recent infection, while IgG indicates past infection or immunity. This distinction is fundamental to serological diagnosis. **Mnemonic:** **"M first, G second"** — IgM appears first in primary response; IgG dominates secondary response and provides long-term immunity. [cite:Kuby Immunology Ch 5]
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