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Antigens are proteins that are found on the surface of the pathogen. Antigens are unique to that pathogen. The whooping cough bacterium, for example, will have different antigens on its surface from the TB bacterium.
When an antigen enters the body, the immune system produces antibodies against it. Antibodies are always Y-shaped. It is like a battle with the army (antibody) fighting off the invader (antigen). A type of white blood cell called a lymphocyte recognizes the antigen as being foreign and produces antibodies that are specific to that antigen. Each antibody has a unique binding site shape which locks onto the specific shape of the antigen. The antibodies destroy the antigen (pathogen) which is then engulfed and digested by macrophages. (Microbiology Online: https://microbiologyonline.org/about-microbiology/microbes-and-the-human-body/antibody-antigen-complex)
Antigens come in different varieties, namely exogenous, endogenous, autoantigens, neoantigens, viral antigens, and tumor antigens to name antigens.
Hen egg lysozyme epitope
IMAGE SOURCE: Creative Commons