What is an antigen?

An antigen is defined as a substance that triggers an immune response. This can be any molecule, such as a protein or polysaccharide, that the immune system recognizes as foreign. When antigens are detected, the body activates various immune mechanisms, including the production of antibodies, to combat these perceived threats. Antigens can come from pathogens like bacteria and viruses, as well as non-infectious substances like pollen or food proteins.

The other options do not accurately capture the definition of an antigen. For instance, a type of bacteria refers specifically to a microorganism and does not encompass the broader category of substances that can provoke an immune response. A hormone, on the other hand, is a signaling molecule produced by glands in the body, and is not involved in triggering immune responses in the way that antigens are. Lastly, a blood type is determined by specific antigens present on the surface of red blood cells, but it is not itself an antigen; rather, it is a classification based on the presence or absence of those antigens. This understanding emphasizes that antigens play a crucial role in the immune system by identifying potentially harmful entities.