Immunogenicity refers to the ability of a substance to trigger an immune response in the body. In the context of vaccines and biologic therapies, immunogenicity is a key factor in determining their efficacy and safety.

When a foreign substance, such as a virus or bacterium, enters the body, the immune system recognizes it as foreign and mounts an immune response to eliminate it. Vaccines work by introducing a harmless or weakened form of the pathogen into the body, which stimulates the immune system to produce a protective response against the actual pathogen. Biologic therapies, such as monoclonal antibodies or recombinant proteins, work by mimicking the natural immune response to a specific target, such as a cancer cell or inflammatory molecule.

However, not all substances are equally immunogenic. Factors such as the size and complexity of the molecule, as well as the route of administration, can affect the ability of a substance to stimulate an immune response. Additionally, genetic factors can also play a role in determining an individual's immune response to a particular substance.