ABSTRACT
Being immune means that the body is able to specifically and successfully react to a foreign material . To be effective, the immune response must be able to distinguish foreign substances, that is, “not self” from “self .” It must be able to remember a previous encounter; that is, there must be memory . In addition, it must be economical; that is, the immune substances should not be produced all the time, but they should be turned on and off, as needed . Moreover, it must be specific for a specific antigen . Immunization, when successful, satisfies all of these properties . Indeed, it is difficult to underestimate the contribution that vaccines have made to our well-being . If there were no childhood immunizations against diphtheria, pertussis, measles, mumps, and rubella, as well as no protection afforded by vaccines against tetanus, polio, influenza, and chicken pox, childhood death rates would probably hover in the range of 20%–50% . Indeed, in those countries where vaccination is not practiced, the death rates of infants and young children remain at that level .
