Antigens are substances that trigger the body’s immune system to produce antibodies. Antibodies then attach to the antigens.
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Antigens can be proteins, polysaccharides (sugar molecules), or nucleic acids (DNA or RNA). Most antigens are proteins, but polysaccharides and nucleic acids also can act as antigens.
The immune system recognizes antigens as foreign entities that are potentially harmful to the body. Some antigens are harmless, such as pollen, while others can cause disease, such as viruses and bacteria.