Hepatitis B virus causes type B hepatitis and it belongs to hepadnavirus family. It is spherical in shape with a diameter of 42 nm. It contains a 27 nm partially double stranded DNA core enclosed within a lipoprotein coat. The antigenic activity of the nucleocapsid core is designated as hepatitis B core antigen. The antigens in the outer surface are called as hepatitis B virus surface antigens. Core antigens are localized within the nuclei whereas the surface antigens are present in the cytoplasm of the infected cells. Antibodies to surface antigens appear in circulation at an early stage of infection whereas the antibodies to the core antigens are detected after several weeks.