Which immunoglobulin class is the most common antibody found in human serum and is responsible for the majority of secondary immune responses?

Explanation

## Immunoglobulin Classes: Serum Abundance and Function **Key Point:** IgG is the most abundant immunoglobulin in human serum, comprising approximately 70–75% of total serum immunoglobulins and is the predominant antibody in secondary immune responses. ### IgG Characteristics | Feature | IgG | IgM | IgA | IgE | |---------|-----|-----|-----|-----| | **Serum concentration** | 7–16 g/L (highest) | 0.4–2.3 g/L | 0.5–4 g/L | <0.001 g/L | | **Molecular weight** | 150 kDa (monomer) | 900 kDa (pentamer) | 160 kDa (dimer in secretions) | 188 kDa | | **Primary response** | Secondary | Primary | Mucosal | Allergic/parasitic | | **Half-life** | 21–28 days (longest) | 5 days | 6 days | 2–3 days | | **Placental transfer** | Yes (via FcRn) | No | No | No | | **Complement activation** | Yes (classical) | Yes (most efficient) | No (alternative) | No | **High-Yield:** IgG is the **only antibody that crosses the placenta**, providing passive immunity to the newborn for the first 3–6 months of life. This is mediated by the neonatal Fc receptor (FcRn). ### Why IgG Dominates Secondary Responses 1. **Affinity maturation**: During secondary responses, B cells undergo somatic hypermutation and class switching, producing high-affinity IgG antibodies. 2. **Longevity**: IgG has the longest half-life (21–28 days), allowing sustained immune protection. 3. **Memory B cell output**: Memory B cells preferentially differentiate into IgG-secreting plasma cells upon re-exposure to antigen. **Clinical Pearl:** In acute infections, IgM appears first (primary response), but IgG rises rapidly and becomes dominant by 1–2 weeks, persisting for years or decades as immunological memory. ### IgG Subclasses (Brief Overview) IgG is further divided into four subclasses (IgG1, IgG2, IgG3, IgG4), with IgG1 and IgG3 being the most efficient at complement activation and opsonization. **Mnemonic:** **GAME** — IgG (secondary, Antibody, Memory, Everywhere in serum). [cite:Kuby Immunology Ch 5]