Antigen refers to something foreign to the host and that causes host immune system cells to make antibody molecules directed against the antigen. Antigen, or Ag , is sometimes used synonymously with pathogen, but actually pathogens possess many antigen molecules on their surface. Foreign to the host, or at least perceived as foreign to the host. A host immune system would of course not attack its own molecules, so antigens must be different chemically from host molecules. Chemically complex .
Proteins are more complex, and make for more potent antigens, than carbohydrates.
Lipids are not very antigenic. Small molecules are not visible to the immune system. Hapten refers to a substance that is foreign to the host, but that does not lead to antibody formation and is not recognized by the host immune system, usually due to being too small or too chemically simple. Drugs are common examples of haptens. If combined with another molecule, perhaps even a host cell molecule, haptens may be seen as antigen and attacked by the immune system.