Definition: Antigen refers to a substance that triggers an immune reaction in humans, specifically in the form of antibodies. The term "antigen" comes from the Greek words for "head," "eye," or "brain," referring to the molecule that binds with these structures and causes an immune response. The term is used primarily to describe proteins found within certain types of organisms (such as viruses) and their antigens, which are specific parts or molecules they present. This definition helps in understanding how antibodies work, specifically in recognizing and neutralizing pathogens like virus particles.
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