Which is the most common structural protein of HIV that is targeted by neutralizing antibodies during natural infection?

Explanation

## HIV Surface Glycoproteins and Immune Recognition **Key Point:** gp120 is the most frequently targeted envelope glycoprotein by neutralizing antibodies during HIV infection because it is the primary attachment protein and is exposed on the virion surface. ### Structural Organization of HIV Envelope HIV contains two main envelope glycoproteins: - **gp120** — surface glycoprotein; binds to CD4 receptor on target cells; highly immunogenic and variable - **gp41** — transmembrane glycoprotein; mediates membrane fusion; more conserved but less exposed ### Antibody Response During Natural HIV Infection | Protein | Location | Exposure | Antibody Response | Clinical Significance | |---------|----------|----------|-------------------|----------------------| | gp120 | Surface | Highly exposed | Primary target | Most neutralizing antibodies | | gp41 | Transmembrane | Partially hidden | Secondary target | Emerges later in infection | | p24 | Core | Internal | Strong but non-neutralizing | Used for diagnosis | | p7 | Matrix | Internal | Weak response | Not neutralizing | **High-Yield:** gp120 is the primary target because: 1. It is the first point of contact with the immune system 2. It is exposed on the virion surface and accessible to antibodies 3. Neutralizing antibodies against gp120 can block viral attachment and entry 4. It is the most variable region, driving immune selection pressure **Clinical Pearl:** The gp120-CD4 interaction is the basis for: - Tropism determination (R5 vs X4 strains) - Vaccine design strategies (most vaccine candidates target gp120) - Entry inhibitor drugs (maraviroc blocks CCR5 co-receptor) **Warning:** Do not confuse gp120 (surface, neutralizing antibody target) with p24 (core, diagnostic marker). p24 antibodies are abundant but do NOT neutralize virus. ### Immune Escape and gp120 Variability The high mutation rate in the gp120 gene (especially in the variable loops V1–V5) allows HIV to evade neutralizing antibodies, making it the primary driver of viral evolution during chronic infection.