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| An antigen or immunogen 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. 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. - Tolerogen - An antigen
that invokes a specific immune non-responsiveness due to its
molecular form. If its molecular form is