Antibodies are usually produced in response to the stimulation of antigens. A person has no antibodies in the body at birth, and the only antibodies that may be present are the small amounts that enter the newborn’s body from the mother through the placenta. After birth, newborns are given the hepatitis B vaccine and the BCG vaccine, and they develop antibodies to the hepatitis B virus infection and will show positive hepatitis B surface antibodies, as well as positive anti-tuberculosis antibodies. However, the hepatitis B antibodies will gradually decay in the body, so after several years the hepatitis B surface antibodies will disappear again and the protection against hepatitis B virus infection will be lost, and the hepatitis B vaccine will need to be re-injected to produce the corresponding hepatitis B antibodies.