Antigens are typically defined as substances that trigger immunogenic response. In general, antigens are proteins, peptides and polysaccharides that originate from within the body or from the external environment. Antigens that enter the body from outside via ingestion, inhalation, or injection are termed as exogenous antigens. These include pathogens, chemicals, toxins, allergens, pollens, etc.
Antigens are a key component of many in vitro diagnosis (IVD).