- The way the body defends itself against substances it sees as harmful or foreign. In an immune response, the immune system recognizes the antigens (usually proteins) on the surface of substances or microorganisms, such as bacteria or viruses, and attacks and destroys, or tries to destroy, them. Cancer cells also have antigens on their surface. Sometimes, the immune system sees these antigens as foreign and mounts an immune response against them. This helps the body fight cancer.