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An antigen is a substance on the surface of a virus that causes a person's immune system to recognize and respond to it. When the body is exposed to an antigen, the body views it as foreign material and takes steps to neutralize the antigen by producing antibodies.

An antibody is a substance found in the blood that the body produces in response to a virus. Antibodies protect the body from disease by attaching to the virus and destroying it. The vaccines help the body to create antibodies that can last for the whole life and provide protection from diseases. Since there are many types of pathogens and antigens that the body needs to protect itself against, there are vaccines available for almost all the known types.

For example,
Hepatitis B Surface Antigen (HBsAg) is a protein on the surface of the hepatitis B virus.The body produces antibodies to destroy this protien and prevents the hepatitis B infection.