The link explains the difference between a protein and an antigen.
Antigen is a protein that can attack mammalian immune system and cause immune response: such as inflammation, fever and finally generation of antibodies.
All antigens are proteins but all proteins are not antigens. For instance, virus can synthesize an enzyme that would participate in genome repair or replication. That will not be an antigen, usually antigens are somewhat toxic to the host or to animal where they are injected for antibody production. For instance, scientists around the world had tried for long time to generate Polio vaccine. Polio Vaccine is nothing but antibodies again toxic protein (antigen) of Polio virus.
Hope this helps.
Naina B. PhD