Antigens are molecules that are found on the surface of pathogens and are specific to that pathogen

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.