Immunogenicity is the ability of a foreign substance, such as an antigen, to provoke an immune response in the body of a human or other animal. In other words, immunogenicity is the ability to induce a humoral and/or cell-mediated immune responses.
Distinction is made between wanted and unwanted immunogenicity:
Wanted immunogenicity is typically related with vaccines, where the injection of an antigen (the vaccine) provokes an immune response against the pathogen (virus, bacteria), protecting the organism from future exposure. Vaccine development is a complex multi-step process, with immunogenicity being at the center of vaccine efficacy.
Unwanted immunogenicity is an immune response by an organism against a therapeutic antigen (ex. recombinant protein, or monoclonal antibody). This reaction leads to production of anti-drug-antibodies (ADAs) inactivating the therapeutic effects of the treatment and, in rare cases, inducing adverse effects.