## Target of Vaccine and Neutralizing Antibodies **Key Point:** The spike (S) protein is the primary immunogenic target of SARS-CoV-2 and is responsible for viral attachment and entry into host cells via the ACE2 receptor. **High-Yield:** All currently approved COVID-19 vaccines (mRNA, viral vector, protein subunit) are designed to elicit immune responses against the S protein, specifically the receptor-binding domain (RBD). ## Structural Role of S Protein The spike protein exists as a trimer on the viral surface and undergoes conformational changes to facilitate: 1. Binding to human ACE2 receptors on respiratory epithelial cells 2. Fusion of viral and host cell membranes via the fusion peptide 3. Cell entry and viral replication initiation ## Why S Protein is the Vaccine Target | Feature | Significance | | --- | --- | | Surface exposure | Accessible to antibodies; highly immunogenic | | Functional role in entry | Blocking S protein prevents infection | | Neutralizing antibody response | Antibodies against RBD prevent ACE2 binding | | Mutation hotspot | Variants of concern (Alpha, Delta, Omicron) have S protein mutations | **Clinical Pearl:** Neutralizing antibody titers against the S protein correlate with protection from infection and severe disease, making it the ideal vaccine antigen. **Mnemonic:** **SPEAR** — S protein is the Primary target, Exposed on surface, Antibody-neutralizable, and Required for entry.
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