An<unk>  (from antibody-generating) or immunogen is a molecule that sometimes stimulates 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.<unk> s are usually proteins or polysaccharides. This includes parts (coats, capsules, cell walls, flagella, fimbrae, and toxins) of<unk> cteria, viruses, and other microorganisms. Lipids and nucleic acids are<unk> ic only when combined with proteins and polysaccharides.  Non-microbial exogenous (non-self)<unk> s can include pollen, egg white, and proteins from transplanted tissues and organs or on the surface of transfused blood cells.
Tolerogen - An<unk>  that invokes a specific immune non-responsiveness due to its molecular form. If its molecular form is changed, a tolerogen can become an immunogen.
Allergen - An allergen is a substance that causes the allergic reaction. The (detrimental) reaction may result after exposure via ingestion, inhalation, injection, or contact with skin.
Cells present their<unk> s to the immune system via a histocompatibility molecule. Depending on the<unk>  presented and the type of the histocompatibility molecule, several types of immune cells can become activated.
Antigen
Substrate 
Allergen
Biomolecule
Phytochemical
Immunotoxin
Enzyme
Ionic liquid