ChEBI

Any substance that stimulates an immune response in the body, such as through antibody production or by presentation to a T-cell receptor after binding to a major histocompability complex (MHC).

Any toxin secreted by bacteria, viruses, fungi or protozoa enabling them to achieve colonisation of a niche in the host, inhibit or evade the host's immune response, enter and exit cells, or obtain nutrition from the host.