“An antigen is a substance that prompts the generation of antibodies and can cause an immune response. The word originated from the notion that they can stimulate antibody generation. We now know that the immune system does not consist of only antibodies. The modern definition encompasses all substances that can be recognized by the adaptive immune system.
Antigens are usually proteins or polysaccharides. This includes parts (coats, capsules, cell walls, flagella, fimbrae, and toxins) of bacteria, viruses, and other microorganisms. Lipids and nucleic acids are antigenic only when combined with proteins and polysaccharides. Non-microbial exogenous (non-self) antigens can include pollen, egg white, and proteins from transplanted tissues and organs or on the surface of transfused blood cells.” - from wikipedia
