Foreign Antigen

A foreign antigen is any molecule or molecular structure that originates from outside the body and is recognized by the immune system as non-self. These antigens are capable of inducing an immune response, typically resulting in the production of antibodies or activation of immune cells to neutralize or eliminate the foreign entity. Most potent antigens are proteins, but polysaccharides, lipids (when complexed with proteins), and nucleic acids can also serve as antigens. Antigens contain specific regions called epitopes that are recognized by antibodies or lymphocyte receptors. Foreign antigens function as markers for pathogens (such as viruses, bacteria), toxins, transplanted tissues, allergens, etc., signaling their presence to the host's immune system for elimination. They trigger both humoral (antibody-mediated) and cellular arms of adaptive immunity. Foreign antigens form the basis for vaccine design: vaccines introduce harmless forms of these molecules to prime immunity without causing disease.