Antigen (an'ti-jen)

Simple - A substance, foreign to the body, that stimulates the production of antibodies by the immune system. Antigens include foreign proteins, bacteria, viruses, pollen and other materials.

An antigen is a molecule that 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 only consist of antibodies. The modern definition encompasses all substances that can be recognized by the adaptive immune system.

Tolerogen - An antigen 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 antigens to the immune system via a histocompatibility molecule. Depending on the antigen presented and the type of the histocompatibility molecule, several types of immune cells can become activated.