Which immunoglobulin class is primarily responsible for opsonization and complement activation in the primary immune response?

Explanation

## Immunoglobulin M (IgM) — Structure and Function **Key Point:** IgM is the first antibody produced during the primary immune response and is the most efficient activator of the classical complement pathway. ### Structural Features of IgM - **Pentameric structure** — composed of 5 immunoglobulin units joined by a J chain - **Molecular weight:** ~900 kDa (largest antibody) - **Valency:** 10 antigen-binding sites (though steric hindrance limits effective binding) - **Half-life:** ~5 days (shorter than IgG) ### Functional Characteristics | Feature | IgM | IgG | |---------|-----|-----| | **Primary response** | Yes (first) | No (secondary) | | **Complement activation** | Highly efficient (one molecule activates) | Less efficient (requires multiple molecules) | | **Opsonization** | Moderate (pentameric structure aids) | Excellent (monomeric, more flexible) | | **Crosses placenta** | No | Yes | | **Mucosal immunity** | No | No (IgA does) | **High-Yield:** IgM is the **earliest marker** of acute infection — its presence indicates recent or active infection, making it diagnostically valuable in serology. **Clinical Pearl:** In the primary immune response, IgM appears first (1–2 weeks), peaks, then declines as IgG takes over. In secondary responses, IgG dominates from the start. ### Why IgM is Superior for Complement Activation 1. Pentameric architecture provides multiple C1q binding sites 2. A single IgM molecule can activate the complement cascade 3. IgG requires at least two molecules in close proximity to activate complement [cite:Robbins 10e Ch 6]