Immunogenicity refers to how effectively a foreign substance — usually an antigen — induces an adaptive immune response in the body. The term is often used as an endpoint in early (Phase 1 and 2) clinical trials of vaccines, but it can also be an endpoint or secondary endpoint in Phase 3 trials. Another context in which reporters may come across the term is trials for immune-related medications, such as immunotherapy treatments. Two types of immunity that can be assessed are how well an antigen leads to a humoral immune response and to a cell-mediated immune response.
Immunogenicity can be assessed in different ways, but likely the most common way is by measuring levels of antibodies.
A type of immunogenicity a person does not want is against a therapeutic agent because that would mean the immune system is attacking a therapy intended as a treatment.
Deeper Dive
A paper in 2015 suggested there was value in splitting immunogenicity into two subcategories of antigenicity and allergenicity. This is getting pretty technical and in the weeds for most reporters, but if you’re doing an in-depth story into allergens and/or the interaction of vaccines and allergies, it may be helpful to think about that distinction, in which case you’ll want to read that paper.
Antigenicity refers to how well the antigen binds to an agent of adaptive immunity, such an antibodies or T cells. It’s possible for an antigen to bind to antibodies or T cells — to have antigenicity — yet not result in immunogenicity, the ability of the immune system to respond effectively enough to create a memory response for the vaccine (i.e., for the vaccine to cause immunity that will work later if the person encounters the real pathogen).
It’s also possible for something to be immunogenic without antigenicity occurring, especially if a vaccine works but scientists aren’t necessarily sure how or why it works. There are several older vaccines where the specific antigens that cause it to be effective are not yet known. In the case of RSV vaccines, it wasn’t possible to develop them until scientists were finally able to figure out what protein properly invoked the kind of immune response needed — and not one that was harmful, as this article describes.
Allergenicity refers to an antigen inducing an abnormal immune response — that is, attacking something that isn’t actually a threat to the body. It’s an overreaction of the immune system, so as allergies in response to pollen.