Antigens are molecules that are found on the surface of pathogens and are specific to that pathogen.
Antigen is a molecule capable of causing the immune system to produce antibodies against it. It may be foreign or formed inside the body. Antigens are usually polysaccharides , lipids or peptides e.g. hemagglutinin ( found in Influenza virus ) .
Pathogen is an infectious agent that may cause a disease. Virus, bacteria, fungus and other microorganisms are often pathogenic, e.g. Influenza virus.