The human immune system is able to defend itself through humoral immunity, of which antibodies are the weapons. A virus or bacteria that infects the body may produce one or more antigens, and the body’s production of the corresponding antibodies indicates that the body is infected with the virus or bacteria, or that it has the relevant immune capacity. Whether or not the antibody is immune depends on the circumstances. Some antibodies are protective, while others are positive and simply indicate a bacterial or viral infection. A positive surface antibody for hepatitis B is a protective antibody that indicates that the person does not have hepatitis B and has acquired immunity to it. A positive HIV antibody indicates a person with AIDS, but does not indicate immunity to AIDS.