Antigens are molecules that can induce an immune response. They are proteins that are found at the surface of a pathogen. Antigens are often foreign, and they are unique to a specific pathogen. When an antigen present to the body, the immune (defense) system produce antibodies against the antigen, the antigen is the molecules that the immune system recognizes and produces antibodies against.
The antigen can be any molecule. It can be a protein, nucleic acid, or polysaccharides. On the other hand, pathogens are organisms or substances, especially microorganisms, that are capable of causing diseases. Most of the pathogens we hear about are bacteria, viruses, protozoa, or fungi. Microorganisms are not considered to be pathogenic until they have reached a specific population size that they can cause disease. I hope you find this information helpful.