Vaccinology — In the licensed recombinant vaccine used against Hepatitis B virus (HBV), the immunogen is produced against which specific HBV antigen to generate protective antibodies?

Introduction / Context:
The modern Hepatitis B vaccine is a landmark of recombinant DNA technology. Understanding which viral antigen is used as the immunogen clarifies how neutralizing immunity is generated and why the vaccine prevents infection and chronic carriage.

Given Data / Assumptions:

• The Hepatitis B virus (HBV) has several antigens: HBsAg (surface), HBcAg (core), and HBeAg (secreted e antigen).
• Neutralizing antibodies that block viral entry target the surface antigens exposed on virions.
• Recombinant technology allows safe production of a single viral protein without whole virus.

Concept / Approach:
Protection against HBV correlates with anti-HBs antibodies, which neutralize virus by preventing attachment and entry into hepatocytes. Recombinant vaccines express HBsAg (typically in yeast), which self-assembles into non-infectious virus-like particles that strongly stimulate protective antibody responses without exposure to infectious virus.

Step-by-Step Solution:

Verification / Alternative check:
Serologic testing after vaccination measures anti-HBs titers; individuals with adequate anti-HBs are protected even if anti-HBc is negative, confirming that surface antigen is the key immunogen.

Why Other Options Are Wrong:

• HBcAg: induces cell-mediated responses but is not the principal neutralizing target for licensed vaccines.
• HBeAg: a secreted protein; antibodies to HBeAg do not confer sterilizing immunity.
• “All of the above” is incorrect because current recombinant vaccines focus on HBsAg.
• HBV polymerase is an internal enzyme and not used as the vaccine antigen.

Common Pitfalls:
Confusing diagnostic markers (HBcAb, HBeAg) with vaccine antigens. Only anti-HBs reliably indicates vaccine-induced protection.

Final Answer:
Viral surface antigen (HBsAg).