Let me interpret the lyrics for you, as best I can. In immunology, an antigen is a large molecule that evokes the production of one or more antibodies by the body's so-called B cells. Each antibody binds to a specific antigen by way of an interaction similar to the fit between a lock and key. These antigens are found on the surface of most viruses, bacteria and other types of nasty microorganisms. The immune system will try to destroy or neutralize any antigen that is recognized as a foreign and potentially harmful invader with the help of these antibodies.
Cancer cells are "self" and thus not foreign, but they're also not normal, so antibodies of a particular configuration are sometimes able to bind to their antigens. Killer T cells that originate in the bone marrow (like B cells they're a type of white blood cell) then come along and destroy the cells that have been tagged. This process is done imperfectly, as cancer cells are often able to evade detection through various ingenious cloaking mechanisms. The video does not depict the immune response of the body to cancer, but rather to infection in general, which explains the reference toward the end to vaccinations.This science video makes an advanced concept of immunology almost comprehensible. The notion of an antibody floating through the bloodstream before binding to an antigen and lysing its cellular contents is something I’ve read about, but could never quite visualize. Now I can. As for that kid who floats by on the boogie board, I’m not sure what kind of cell he’s meant to represent. A macrophage, perhaps?