People who become infected naturally with hepatitis A acquire neutralizing antibodies against the virus. In the U.S., hepatitis A has been a common, and often asymptomatic, infection. Therefore the prevalence of antibodies rises with age:

Because of this seroprevalence, pooled immune globulin from U.S. blood donors can be used for passive immunization of persons exposed or "at risk" for hepatitis A.

If you immunize with human immune globulin or hepatitis A vaccine (HAV vaccine), you can measure specific antibodies in the recipients, as shown below: