An antigen is any molecule against which a specific immune response can be generated. Molecules differ in terms of how well they activate lymphocytes to produce this specific response.
Foreign substances tend to be immunogenic.
Molecules which are chemically complex are immunogenic. Therefore foreign proteins and carbohydrates are good antigens. Virus particles and Bacterial cells are composed of many antigens. That is because bacteria and viruses are composed of many different molecules which are foreign to the human body. Each antigen, for the sake of convenience lets say each protein antigen, has on it several antigenic determinants .