Figure 8: Scheme of the immune system response (immune effectors producing antigen-receptor a and immune-memory cells ) to an invading pathogen (pathogen carrying antigen ). Two-state model: immune effector is turned on (from neutralized state to effector state) when average number of pathogens go above threshold ; pathogen is turned off (from proliferating to neutralized state) when average immune response goes above threshold ; immune effector is turned off (from effector state to neutralized state) when average number of pathogens go below threshold. First infection by the proliferating pathogen (high multiplication factor, high selection probability to survive) initiates the immune response of the host: an immune cell with the recipe for antigen-receptor “” is singled out from the reservoir of naive immune cells to form an immune effector which proliferates (high-multiplication factor, high-selection probability). Above threshold titer of antigen-receptor , the pathogen is neutralized (low multiplication factor, low selection probability). The immune effectors are transformed into immune-memory cells which do not produce antigen-receptor a but carry its recipe (low-multiplication factor, high-selection probability). During any further infections by the pathogen , the immune-memory cells with the recipe for antigen-receptor are formed back into immune effectors producing antigen-receptor . Pathogen carrying antigen can transform to pathogen carrying antigen . A new immune respond has to be launched with immune effectors producing antigen-receptor and immune-memory cells .