Antigens are usually macromolecules or macromolecular complexes that contain distinct antigenic sites or “epitopes,” which can be recognized and interact with the various components of the immune system. They can exist as individual molecules composed of synthetic organic chemicals, proteins, lipoproteins, glycoproteins, RNA, DNA, and polysaccharides—or they may be parts of cellular structures (bacteria or fungi) or viruses. https://www.creative-diagnostics.com/food-analysis/tag-antigen-18.htm