HBcAg is the core antigen of hepatitis B. The core antigen exists only in the hepatitis B virus, so only when the hepatitis B virus infects the body and replicates within the body, the body’s immune system may come into contact with the core antigen and thus produce core antibodies, so it means that the body was or is infected with the hepatitis B virus. Therefore, it is a sign that the body was or is infected with the hepatitis B virus. However, if the core antibody alone is positive, it is also possible that hepatitis B has been cured, because after the body is infected with the hepatitis B virus, some people will clear the hepatitis B virus through the immune system, and after the clearance, the core antigen of hepatitis B is gone, but the core antibody will still exist in the body. However, the core antibodies are still present in the body. Therefore, it is necessary to combine the results of the five hepatitis B tests with other results, such as surface antigen or surface antibody, to distinguish the difference.