Regarding the structural and functional properties of different immunoglobulin classes, all of the following are true EXCEPT:

Explanation

## Correct Answer Analysis **Key Point:** IgD is NOT the major antibody in secondary immune response. **IgG is the predominant antibody in secondary immune response**. IgD serves as a B-cell surface receptor and is involved in B-cell activation, but it is not a major secreted antibody in any immune response. ## Immunoglobulin Classes: Structure and Function | Ig Class | Molecular Form | Location | Primary Role | Key Feature | |----------|---|---|---|---| | **IgG** | Monomer | Serum (75% of Igs) | Secondary response | Crosses placenta; opsonization; ADCC | | **IgA** | Dimer (secretory) | Mucosal secretions | Mucosal immunity | Most abundant Ig overall | | **IgM** | Pentamer | Serum | Primary response | First antibody produced; complement | | **IgE** | Monomer | Serum (trace) | Hypersensitivity | Binds mast cells/basophils (high-affinity FcεRI) | | **IgD** | Monomer | B-cell surface | B-cell activation | Minor serum levels; not major response antibody | ### Why Option 2 Is Wrong **High-Yield:** IgD is primarily a **B-cell surface receptor** (BCR co-receptor with IgM) and plays a role in B-cell activation and tolerance. It is **NOT the major antibody in secondary immune response**. The major antibody in secondary response is **IgG**, which appears after IgM and dominates the response due to class switching and affinity maturation. **Clinical Pearl:** IgD levels are very low in serum (~0.03% of total immunoglobulins) and it has minimal effector function. Its main role is signaling on B cells, not providing immunity to pathogens. ## Temporal Antibody Response ```mermaid flowchart TD A[Antigen Exposure]:::outcome --> B{Primary vs Secondary?}:::decision B -->|Primary Response| C[IgM First]:::action C --> D[IgG appears later]:::action B -->|Secondary Response| E[IgG Dominant]:::action E --> F[Higher titer, faster kinetics]:::action G[IgD on B-cell surface]:::action G --> H[B-cell activation signal]:::action I[IgA in secretions]:::action I --> J[Mucosal protection]:::action ``` **Mnemonic:** **IgG = Secondary** — Remember that IgG dominates the secondary (and subsequent) immune response due to class switching and somatic hypermutation.