Antigenicity refers to the ability of a substance, known as an antigen, to provoke an immune response in the body. Antigens are typically proteins or polysaccharides found on the surface of pathogens like bacteria and viruses, as well as on allergens and even some cancer cells. When the immune system recognizes these antigens as foreign, it activates immune cells to attack and eliminate them. The strength of an antigen's ability to elicit an immune response can vary based on its structure and the host's immune system. Factors such as genetic makeup, age, and overall health can influence how effectively an antigen triggers immunity. Understanding antigenicity is crucial for developing vaccines and therapies to combat diseases.