Resistance of vertebrate hosts against lethal infections depends crucially on adaptive immune responses of T and B cells. Viruses, bacteria and parasites all probe the limit of immune responses and of immunity. I shall document how low-affinity antibodies measured by ELISA obey drastically different rules than high-affinity protective (neutralising) antibody responses. Similarly, effector T cell specificity measured as cytotoxic CD8+T or IgG-helper CD4* T cells differ from (”low-affinity”?) T cells measured by IFNg release, tetramers or proliferation. Immunity, i.e. protection against lethal infections must ultimately be the key to assess strengths and limitations of immune responses to understand “specificity” and thereby also “tolerance” and “memory”. Any other measurement can be too easily misused to show what we want to show and not what immunity is all about!