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An antigen is any substance or organism foreign to the body that induces an immune response. This response includes the formation of one or more antibodies, as well as various cellular immune responses involving T- and B-cell lymphocytes. Under ordinary circumstances, it must be of a certain minimum molecular size in order for antibody formation to occur.
Antigens include infectious agents such as bacteria, viruses, and mycoplasma or chlamydia organisms, as well as chemicals, food proteins, pollen grains, dust particles - you name it. If the specific type of immune response induced by the antigen involves the allergy-mediating part of the immune system, the antigen is referred to as an allergen.